Number 1 reason in America is bad console ports. Alpha 3 probably would have been still popular to this day if it got a good PS2 port. It never got a PS2/OG Xbox era port until after 3rd Strike came out and it still was rather off. When Alpha 3 was really popular the FGC was still mostly arcade based and by the time Evo and tourneys started to go console based there was still no good console port. CVS2 was generally starting to take over as the next biggest game to 3S since it had a better console port (both PS2 and Dreamcast were pretty close to arcade).
Alpha 2 from what I’ve heard was pretty niche competitively in Japan compared to the US and was pretty much dumped once A3 hit. A3 was the BIG SF game in Japan for a while, then eventually 3rd Strike also got pretty big right after with SBO and such.
That’s pretty much why in the US when Evo started rolling around A2 and A3 only caught some steam online because of GGPO. Both games online had a bit of a community there, but I can’t imagine enough people wanted to play PC emulated ports of those games at offline tourneys. Evo and other majors mostly stuck to consoles so that didn’t help.