Not sure if it's frequency bias, but when did the perceived "Poker == Texas Holdem" happen? I remember when I was learning about Poker as a kid/teenager in Europe I didn't hear about that variant, then I didn't play for a bit and suddenly there's only Texas Holdem.
My guess would be during the US poker boom around 2003ish. Texas Hold Em had for quite a while been the variant of poker used by the largest tournaments, and during that boom a lot more people learned about competitive poker and started playing it online.
As for why Texas Holdem is the most popular variant for betting poker, it’s a variant that is not too complex, offers a lot of opportunities to infer information about other player’s hands due to minimal private information compared to other variants, and has multiple opportunities for betting. It works well from both a competitive and watchability stand point.
I certainly can't speak in general but I noticed it enter my circles of friends when the World Poker Tour and Celebrity Poker Showdown games started on TV ~2003. Around that time it seemed like everyone I knew was suddenly interested in Texas Hold'em (before that I had basically only ever played 5-card draw).
