It's far less studied and didn't exist (to any usable degree) at the time it was made. And it's extremely different, organizationally, so it's a difficult migration to pull off. And it has different incentives. And most miners have a lot of costs (e.g. hardware) that won't be recoverable if it's changed, so there's a fairly strong incentive to not change it.

Ethereum uses proof of stake now though (since 2022). Which happened in part because Ethereum is effectively centralized, or at least significantly more-so than Bitcoin.