Because before 2008 there wasn't enough euro or dancey sounding hits with house patterned beat styles or "hard enough" euro sounds to make it on the American side. We were still in the "building up process", but not quite there yet. It was only after very late 2008 when things officially began to take off and people were REALLY getting into the harder dance and house style instrumentations, while at the same time, taking it up a notch with the bpm's (which also led to many more rhythmics and top 40's popping up as the urban sound started to die off).
Let me go back to my early days of when I first started saying dance music could make a come back and how it could be done. It all started back with my "DMC THEORY" (Dance Music Comeback Theory) that I introduced to people around 2005 and 2006. I started telling people all we have to do is have someone well known use a house, booty bass, or even breakbeat or freestyle type beat, do it the right way, and become successful with it in the mainstream, then BAM! A bunch of people would imitate the style, copy it, and follow along. I also mentioned how we needed people to be more creative and do collaborations that people would never expect with artists from different genres teaming with hip hoppers...
THIS IS PRECISELY WHAT HAPPENED!
Missy dropped "lose control" which had a freestyle / break beat and rapped on it sort of in a booty bass style. It was successful. This lead to a L.L. Cool J production doing "control myself" with Jennifer Lopez, which also had a freestyle type breakbeat as well! Shortly after, to add on to the break/bass sound of things, came the Lil Jon remix of My Humps...etc etc etc. (and you can continue to flash forward to Fergalicious, Ciara's "get up", Flo Rida's "low"...etc etc etc). There you have it, the booty bass sound was officially making it's come back. AROUND THE SAME TIME Missy re-jumpstarted the bass/break sound in the mainstream with "lose control", Timbaland successfully dropped "Sexy back" with Justin Timberlake, which had a house pattern beat. This eventually lead to the Britney "Gimmie More", "the way I are" by Keri Hilson", and Pitbull "go girl" etc etc etc sound. As time went on, we continued to evolve that house pattern beat style and make it harder and faster (leading to poker face, Chris Brown's "forever", and eventually Ne-yo's "closer"...etc). This all meanwhile the booty bass/break sound is also evolving and getting harder as well. Eventually, we got to the point where Lady Gaga's style and sound could be easily accepted and no longer was "too weird or different". Ironically, this just happened to be around the same time Akon took the break/bass style to another level with "beautiful", which was a straight up euro break (or even new school freestyle) track.
IT WAS OFFICIAL AT THIS POINT THAT DANCE WAS SURELY MAKING A COMEBACK.
So 2008 ends and 2009 begins. Little do we know what Kid Cudi was about to do, or even David Guetta . THIS is the year you knew hip hop was officially done dominating the charts and a whole new generation of sound was going to become the next big thing. This also answers the question why more euro didn't appear on AT40 before 2008. Before 2008, we weren't exactly there yet. We were still on the soft, watered down "sexy back" sound in the mainstream house side of things, and of course the booty bass/break sound wasn't really helping euro because the booty bass/break sound is an American thing and is an urban style, not really electronic. It was only after the house pattern beats pop hits evolved into actual house or very house leaning pop hits that it was "safe enough" to "go euro". It's just that since the day of "Sexy back" and "lose control" being released, we had to sit back and wait for the evolutionary process in the dance sound to intensify, which meant sitting through "gimmie more" by Britney, Rihanna's "don't stop the music", Lady Gaga's "Just Dance"... stuff from Fergie, LMFAO, Nelly Furtado, Colby O Donis, Akon...etc etc etc etc. all which were watered down versions of dance music getting people prepared for ad used to the sound that was about to come...
FLASH FORWARD TO TODAY
And we've come a long way since the day when Missy's "Lose Control" was pretty much the only uptempo hit on maintream radio with a breakbeat, and Justin Timberlake's "sexy back" was the only uptempo hit with the house patterned beat. Now you see how both the house and freestyle/booty bass breaks sound has modernized and come back in mainstream with songs like "down" by Jay Sean, "Gettin over" by David Guetta, "beautiful monster" by Ne-yo, OMG by Usher #1 ON THE CHARTS!, People like Three6Mafia and Tiesto collaborating....etc.
It all started with ONE person who wasn't afraid to try something different and be successful with it during the time when hip hop was still dominating and pretty much blocking everything else out on the charts to bring back a whole new generation of dance sounds in the U.S. The same way it took Noreaga & Nina Sky to mess around with reggaeton and start a whole reggaeton trend in the U.S. to the point where latin rhythmic stations and syndicated shows strictly dedicated to reggaeton started popping up! Even MTV had to add in a reggaeton segment for a while!
SO
I figured if this could happen with reggaeton, then of course it could be done with dance, too! ...And look at us now! 5 years ago today, La Roux, David Guetta, LMFAO, the current Akon and Pitbull sound, and possibly even Flo Rida would not have had a chance. If it weren't for someone to rejumpstart this entire dance sound, Rihanna would've probably only had ONE successful uptempo hit - which would've been "sos" that DJ's would only be able to mix with Britney's recurrent "toxic" before having to spin back the record and change back to a lower tempo. Pitbull COULD have been the one to restart the electronic sound, but unfortunately, as much as he flirted with it, he still kept it watered down and waited for someone else to officially REALLY get uptempo before showing his true colors. Same with Akon.
With that being said, I also have to say one final thing. I give it up to Timbaland and Missy for changing the sound of music TWICE!
FIRST: with the late 90's futuristic beat pattern sounds that was all kicked off with Aaliyah's "one in a million" and a little later "I can't stand the rain" by Missy (which lead to the whole late 90's Destiny's Child, Total, TLC, 702, and even Juvenile, Jay-Z...etc and many others - including influencing the late 90's Neptunes sound) and
SECOND: trying an uptempo sound while everyone else was still slowed down in bpm's, trying to to "urbanize" themselves. If it weren't for the SECOND part of all this to happen, Britney would've never been able to return to her pop roots in an electronic style. I also give credit for all the dumb hip hop hits that started popping up ever since "laffy taffy" and "in my white t / oh i think they like me" in addition to the death of the old school Timbaland beats sound that helped to kill hip hop, which also benefited in helping make room for the electronic sound. I knew once "Ay bay bay" and all those rock yo hips and hip hop dance movement songs that talked about nothing started taking over the hip hop side, it was over for hip hop.
Bottom line, whether or not you see what causes the trends or changes in every situation, things change, and that's just the way it is. Luckily, this change is benefiting the dance sound! Reggaeton COULD have stopped this from happening as soon as it did.... Kiss Kiss by Chris Brown COULD have restarted that whole old school Timbaland beats style sound and helped r&b go off into another new direction... but it just didn't work out that way. The uptempo sound is what stuck this time, and now dance music has a brighter future to look forward to 2010-2019.