Why Does the PlayStation Street Fighter Alpha 3 Have More Characters Than the Arcade?
The arcade Street Fighter Alpha 3 has 28 playable characters. The PlayStation version has 34. The difference is not random. Six genuinely new fighters arrived in two deliberate groups, and a third group of arcade fighters was promoted from hidden to regular. Each change tells you something about what Capcom thought a home port was for in 1998.
The short answer is that the PlayStation version was a second development pass on the game, not a conversion deadline. Capcom used it to finish the roster the arcade schedule had left incomplete, and to give home players secrets that could only be earned in a mode the arcade never had.
THE TWO COVERS
The expanded home versions

THE CHANGES
Three promotions, three completions, three secretsWHY CAPCOM DID IT
The home version was the productNothing about the additions was forced by the hardware; if anything, the PlayStation fought back. Capcom replaced hit sprites with hit polygons specifically to free memory for character animation, and the console's RAM still limited what modes like Dramatic Battle could do. The extra fighters were a choice about what a home port should be.
GameSpot's contemporary review states the intent plainly. Capcom, it said, bent over backwards to deliver its most rewarding home conversion yet, in a show of bravado reminiscent of Namco's Tekken series, whose PlayStation ports were famous for outgrowing their arcade originals. The review's closing line reads like the strategy in one sentence: the hope that Alpha 3 would start a trend of console games exceeding their arcade predecessors. An arcade operator sells minutes; a home publisher sells a package a player keeps, and in 1998 the way to sell that package was more fighters, more modes and secrets worth unlocking.
Capcom eventually ratified the additions itself. When the game returned to arcades in 2001 as Street Fighter Zero 3 Upper, on Sega's NAOMI hardware, it included all six characters from the home console versions. The PlayStation roster had stopped being the port's bonus content and become the game's roster.