As everyone else has said, Street Fighter 2 basically created the template for modern fighting games. It can't be understated just how popular the game was when it came out, both in the arcades and on consoles.
Street Fighter 3 already had an uphill climb due to how massive SF2 was, and a number of the creative choices (sticking with sprites when 3D was becoming more prominent, ditching most of the legacy characters aside from Ryu and Ken) made it even more of a disappointment in fans' eyes. Hindsight has definitely been more kind to SF3 since then, but at the time, it almost killed off the franchise.
"You need a hobby. I suggest sticking forks in your eyes."-DJosef
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