Ethereum's social contract has evolved to become much more "fluid" and open to change than Bitcoin's. Mainline Ethereum today is is very different in many ways to what it was two years ago. There is even a proposal with a lot of support to have transaction fees mostly burned rather than mostly given to miners. Aside from minority forks, something like that would never happen in Bitcoin in a million years.

So, while Bitcoin techinically could switch to proof of stake, it probably won't.