Bitcoin is using a different hash function. Bitcoin uses sha, which is easy to implement more efficiently in fgpa and asic, which blows gpus out of the world.

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.