Because they are weaker, they tend to be faster. Past the trait of being able to crush infantry, speed is a vital consideration to doing so as the vehicle needs to move fast enough while turning to manage to hit into the infantry them as they play their animation for dodging to the side to avoid being crushed while the vehicle approaches. The faster it moves, the easier it is to crush infantry because then it barely has to turn into infantry dodging to run them over.
Thanks sir for the enlightment