Because a proton has A LOT more mass than an electron. Newton's law F=ma indicates acceleration is related to the mass of the object having the force exerted on it. Since the force is the same in both cases, the acceleration must be lower for the proton because it's mass is higher. I'll show an example where the numbers are no where near real, but it will show the relationship:
therefore, a = 5
Notice how (m) times (a) must equal the same value in both cases? That's what's causing the electron to move so much faster, because it's so much less massive.