That's not true at all. They got airplay on rock stations before there was a rock chart. So there was no measurement data on the airplay they received. The Billboard charts mainly looked at Top 40 airplay. Sex Pistols were not a Top 40 band. So they didn't chart in the US. But that doesn't mean they didn't get airplay. Just that we don't have documentation on how much. But their debut album is double platinum, so it definitely got lots of airplay.
Same thing with AC/DC. Their 70s music wasn't released in the US because they were on a small Australian label. Then they signed with Atlantic, teamed up with producer Mutt Lange, and recorded Highway To Hell. The Mainstream Rock chart came about in the early 80s, but once again, they weren't a singles band. They released albums, those albums were services to AOR rock radio, and those stations would play tracks from the albums.
The Sex Pistols got no airplay, at least here in Seattle. The singles were import only, for one thing. When the album came out it was import only for a couple months, and when finally released in the US it made it as far as 106 on the Billboard 200.
And local rock radio (KISW & KZOK) wouldn't touch the album with a ten foot pole even after it was released stateside. Some few alternative stations of the era may have played a track or two, but I listened to such stations locally all the time (along with the two or three AOR stations) and never heard the Sex Pistols played. They probably did eventually get played on some college stations. I'm sure KCMU, where I worked for three years, played them from time to time, although I never personally heard them on that station. By the time I was there the LP was ten years old.
FWIW, I have NEVER heard the Sex Pistols played on a rock station, either here in the Seattle area, or during travels elsewhere in the US (mainly the South, and Oregon / Northern Cal).
What you may have heard was after the breakup of the group, maybe during the 80s some alt-rock stations played one or two of of their tracks as 'gold', as the alternative acts that rose with the grunge/alt movement frequently cited the Sex Pistols as an influence.
RE; AC/DC: They were signed to Atlantic in the US in 1976, which is when the first LP was released here, and radio wouldn't touch their first three US albums (High Voltage, Let There Be Rock, and Powerage) with a ten foot pole. Even calling in and making a request would give you a polite, but downright refusal. It wasn't until their fourth US album -- the live 1978 album that preceded Highway to Hell, "If You Want Blood You've Got It" -- that they got airplay, in Seattle at least. I think that only got airplay because the word of mouth, and groundswell of support came from the constant touring by the band.
Highway To Hell, which
did get airplay from its release in 1979, was AC/DC's first successful LP here. That was when fortunes began to change for the band. But before then? They were basically a cult act, with followings in Jacksonville and a couple other markets, but even in Seattle they weren't played until the bandwagon had already started rolling. And, of course, after 1980 when Back In Black hit #1 on the LP charts, radio stations "discovered" the older stuff, and those older albums began to sell.
So my point is that even 'good' music -- or music that the public eventually can deem good, or 'hit' music -- doesn't always make it to the airwaves, because, as you probably well know, there are limited airplay slots, and so many new releases to choose from. And a lot of what gets on the air is more due to other factors as opposed to actual quality, although what makes something have 'quality' is
always subjective. Disco Duck was a #1 hit in 1976 and although it was a #1 smash hit, it hasn't aged as well as
High Voltage, a record from the same year that radio snubbed for almost 5 years. But, obviously, people loved Disco Duck. Those sales weren't imaginary.
Those of us in the sidelines who make opinions about what should be a 'hit', and what shouldn't be, are like commenters from the peanut gallery -- it's cheap and easy to cast aspersions on radio programmers, but it's not like they have an easy job choosing what to play.