Blitzcrank's passive gives him a large shield when get gets low on a 90 second cooldown. Pyke's passive gives him sustain through poke. One is good for surviving burst damage, while the other is good for lane sustain.
Blitzcrank's Hook can pull enemies farther than Pyke can. Pyke pulls a set distance, so a max-range harpoon won't actually pull the enemy all the way to you. Blitzcrank pulls the enemy the entire length of the skillshot, making it much more effective for catching enemies out. Blitz's hook is also hard CC (a stun) with no charge up time, but Pyke's is just a slow and needs a moment to charge. It also deals more damage than Pyke's harpoon.
Overdrive gives Blitzcrank not only more movement speed, but more attack speed as well. He is able to attack freely while it's active. Pyke might get stealth, but he can't attack without breaking the stealth and losing all of the movement speed.
Blitzcrank's knockup is an empowered auto attack; you can't avoid it. Pyke's is a reverse skillshot. It gives the enemy a warning and the ability to dodge it.
Blitzcrank and Pyke have different ults entirely. Blitz can deal a ton of damage to the entire enemy team and interrupt their abilities, while Pyke can blink around finishing off enemies. Blitz also gets a sizable amount of sustained damage from the passive component from his ult. One is sustained damage and CC, one is an execute. Blitz can start a fight, Pyke ends it. They're not really comparable.
It's also worth mentioning that Blitzcrank can actually build health. He can actually get tankier as the game progresses, where Pyke is always stuck at his base value of health. Pyke is always squishy.
They are different champions, even though they do similar things. Blitz doesn't need to get removed just because Pyke exists.