I'm sure this is a stupid question, but I'm going to go for it anyway: isn't Bitcoin also proof-of-work? Why isn't that where ETH miners are going?
Eth thought that was a bad thing, so chose an algorithm that was intentionally difficult to implement in asic. They thought having gpus be the most efficient platform would be more decentralized and be more one person one vote compared to asics. As a result people use gpus for eth.
Password hashing algorithms (scrypt, argon2, etc) are generally designed to minimize the advantage custom hatdware gives.
Mining BTC on a GPU isn't profitable.
You need a dedicated ASIC chip that is far more efficient at the task than a GPU if you want to hope turning a profit mining Bitcoin.
Something like this: