Because for some reason, SF is the only game where people will actually get mad if the game gets TOO adventurous.
KI can add anybody they want, no one is ever going to say "this doesn't feel like Killer Instinct anymore!" Arc added an absurd character like Jack-O to GG, and no one will say "this doesn't feel like Guilt Gear anymore!"
But, the minute Capcom adds Laura, people flip out. "This doesn't feel like Street Fighter anymore!"
Yet having said that, SF5 does have a lot of unique mechanics for a SF game. Setting traps, strikers, poison, freeze mechanic, double jumping, time bombs, flight, unusual control schemes (Ed), Tager-like vaccum suction (Gief), Aegis, running, chain combos, OTG combos, combo-able throws, storing moves with independent meters (Juri/Ibuki), parry, meter/life stealing, stance switching, alpha counters, attack reversals (Kolin)...
This is the first time so many mechanics have been crammed into a single SF game, and miraculously, the game is pretty dang balanced.