Tyron_Leung
For the '87 cartoon, I think it just came down to what would be most appealing to children and sell the most toys. They wanted to sell Shredder toys in perpetuity, and so all of the other cool villains in TMNT were sidelined or changed in some way to be subservient to Shredder.
A goofy Doc Brown character who mutates into a fly was likely deemed more appealing for the target child audience than just a human genius villain character.
And I'm gonna be honest. As a kid, I thought the fly version of Stockman was the coolest thing ever. It wasn't until I got older that I appreciated the OG Stockman more.
It's a shame that the '87 changed so much from the source material, but they're different enough that I don't even think of them as the same character in my head. It's like a different character just with Stockman's name.
