I think that the real deal of blitzcrank its the .5 sec AoE instant silence that he can pull of once he grabs someone, or someone dives in (its a combo destroyer, the difference between life and death in lategame may be that .5 sec window). Thresh has lots of cc but no AoE silence (The Box is pretty good, but dodgeable).
Moreover, Blitz is not tanky as Thresh, but he can bait with his passive, and the blitz hook if landed well can isolate someone from his team and posibly make him 1v5 your team. If Thresh activates his hook, he will dive towards the hooked enemy and sometimes this can lead to a bad position and a miserable death for him.
I still think Thresh is stronger and he deserves the ban slots if his team knows how to play with him (the lantern toss is a gamechanger). But its ok to ban blitz if you don't want to be constantly aware of your surrondings just in case you get pulled and die, or you want to use an assassin or another combo-champ and have more chances to successfully do your thing.