Blitzcrank has a far superior kit to Nautilus.
Blitzcrank's Q pulls the target TO you with no downsides
Nautilus's Q pulls himself and the target together, and Nautilus could be focused down easily with no mobility left to get out.
Blitzcrank's Mobility is also way better than Nautilus's
Blitzcrank presses W, gets a large speed boost for a bit of time, then gets a very short slow debuff
Nautilus has to SACRIFICE his MAIN CC ABILITY to get out of situations
Nautilus's W is also a mini shield with a small DOT effect
Blitzcrank's E is an instant Knockup that ignores flash, dashes and teleports.
Nautilus's E is an aoe slow, doesn't restrict anything
Blitzcrank's R is an AOE TEAMSIZED SILENCE THAT BREAKS ANY AND ALL SHIELDS, THEN DEALS DAMAGE
Nautilus's R is a targeted knockup that can be dashed away from and be safe while it knocks you up.
Blitzcrank's passive is a Shield that uses 30% of his CURRENT MANA to gain a shield equal to the amount of mana lost upon reaching 20% health
Nautilus has a small stun that has a cooldown per unit.
Pull you into his team, knock you up, then silence you. You're 100% dead because you're literally surrounded by the enemy team.
Teamfighting: Q E someone, they die, W into the enemy team and silence everyone while breaking all their shields
Pull yourself into the open with an enemy, then get targeted down because you used your mobility spell to engage.
Pull enemy, shield, slow, R, AA, that person is dead but you can't do ANYTHING for the rest of the fight because of your cooldowns.
Blitzcrank is so much better to Nautilus in every way, why would ANYONE pick Nautilus.
And don't say "Because Nautilus is a tank and Blitzcrank isn't", Far better tanks with the better CC than Nautilus exist and Blitzcrank also gets extremely tank with 6 items and 600+ shielding every 30s.