WPLJ was definitely an all out CHR/Pop station back in 1989 - it was known as Power 95 back then.
WNCI Columbus has had huge numbers in part b/c its got one of the best signals in the country. Compare it with that most of the other Columbus FM signals and see what I mean. It's also heritage CHR, and spent a good number of those years as an Adult leaning CHR.
As for Indi, it didn't have a lot of signals for a market of its size even in the early 90s, and 99.5 ZPL was a very white bread Midwestern CHR when it held those double digit numbers. It even had problems playing number one hits like EMF's "Unbelievable", and was still playing Eddie Money's "Shakin" in the early 90s. That all changed when Hoosier 96.3 came on the scene with a Rhythmic/Dance format.
As for Country music, its usually had a very hard time crossing over into CHR radio. That was definitely the case in the mid 80s.
Currently the big crossover is Carrie Underwood, of American Idol fame. Her song is even Top 10 at Z-100 NYC.
Anyways, the facts are most markets back in the day only offered Full Service Radio, MOR, and Top 40. If you were lucky, you also had a "Black" station or two in your market, but of course many major cities didn't have one for a long time. So what if there were 2-3 Top 40 and MOR stations! Compare that with the plethora of formats you could have available today. Hell, just on the A/C side of things you have Soft A/C, Bright A/C, A/C Gold, Rhythmic AC, Modern AC, Oldies, Jack/Adult Hits, Christian AC, Spanish A/C, Urban A/C, and of course Hot A/C. For Rock you've got Active, Modern, Classic Rock, Triple A, and College Rock. For CHR you have CHR/PoP, Rhythmic CHR, Adult CHR, CHR/Dance, Kids CHR/Radio Disney, Spanish CHR, and Rock leaning CHR (such as Sirius Hits 1). For Country there's Mainstream Country, New/Young Young, Country Gold, Texas Country, etc. For Talk you've got News/Talk, Conservative Rock, Young FM Talk, Sports Talk, etc.
Today's Top 40 radio has a much more competitive landscape to deal with. The FM dial isn't made up of 10 CHR and 10 MOR stations - it's made of a variety of formats that cater to different segments of the population. Since Top 40 radio is a Current hits based format, its only common sense that its primary focus isn't on the older audience...