I wasn't into AR's at the time due to lack of funds, but as I recall, AR's were something of an enthusiasts' rifle by the early '90s. They were common enough to be occasionally seen at the range, and our local Walmart carried them, but they were *very* expensive and there weren't many options; I knew a guy with a couple of Colt 20" HBARs around 1989, but they were ~$1K, were very front-heavy, and your optics options pretty much sucked. SKS's were all over the place and AK's were reasonably common, but the AR was somewhat elite due to its price point. In perspective, a $1K AR was the equivalent of 14 $69.95 SKS's, or 3 mini-14's or AK's.
I'd say the surge in popularity began in the early '90s due to the Feinstein/Schumer mudslinging and really took off after 1994. I think it was in the mid 1990's that the "M4gery" configuration became dominant, and flattops followed soon after. I'd say by the time the 1994 AWB non-ban expired in 2004, if the AR was probably in the top 3 selling centerfire rifles in the USA and may have even been #1, I don't know.
One thing that happened in the '90s was that the market exploded with new manufacturers. In 1990, I can't remember any other manufacturers besides Colt, but that number had increased to 30 or so by 2004.