Why does blitzcrank speed boost has a drawback

Because neither of them have one of the strongest initiations in the game. Udyr is mostly single-target damage meant to chase and pin down someone to help peel for the team. Teemo is meant to mentally mess with the team using a combination of subterfuge, assassination, and manipulation to bait out people into favorable situations. Blitzcrank is meant to secure an advantage by landing his pull to turn the teamfight into a 5v4. Also, Blitzcrank's pull is stronger than Thresh's and has less counterplay. Thresh's pull not only has half a second of delay and takes time to pull you in, but it can't pull you through walls. Whereas Blitzcrank's pull is an immediate positional displacement if it lands, which pulls you through obstacles unlike Thresh's pull.

The self-slow is one of the few meaningful ways to limit Blitzcrank without having to change his core identity, and also makes sense to add it: he boosts himself to quickly put himself close enough to an opponent to make a play, but he now can't just mindlessly boost himself, instead he temporarily removes his built-in limits but suffers the results of going past them. Now, his boost is meant to be used strategically, in that he has to make a play in the time he has the boost, if he can't then he self-slows and loses the chance.

He can have it changed to no drawback if his hook becomes a melee ranged slow.

+ MS quints + masteries

slows reduced by 35%, two more speed boosts, and very high MS

i call this build the angel of death

sometimes i get two tears and iceborn gauntlet instead of TF and nuke people while having a mana shield the size of canada

Because it's a quick re-positioning tool, not a gap closer due to how powerful your Q can be.

If you want to talk kits, Teemo doesn't have a knock-up, silence, and can't drag you into his entire team with one skillshot. Udyr can stun, but doesn't get a skillshot to grab you, he has to run at you to do it.