I don't know enough to answer "what mechanisms of how the AI works would cause it to be worse than Terran and Zerg."
If the question is rather "what characteristics of Alphastar's Terran and Zerg play style make me say that its Terran and Zerg play is worse than its Protoss play," the simplest answer is that Alphastar just feels a lot more like a bot. Unlike when playing Protoss, it seems to get into certain "ruts" of unit composition and tactics that are a bad match for the opponent it's facing and can't seem to reactively change based on the game is going, whereas with Protoss it seems more than happy to change its play style over the course of the game based on what the opponent is doing.
If the question is rather "what characteristics of Alphastar's Terran and Zerg play style make me say that its Terran and Zerg play is worse than its Protoss play," the simplest answer is that Alphastar just feels a lot more like a bot. Unlike when playing Protoss, it seems to get into certain "ruts" of unit composition and tactics that are a bad match for the opponent it's facing and can't seem to reactively change based on the game is going, whereas with Protoss it seems more than happy to change its play style over the course of the game based on what the opponent is doing.