It's a weird variation on network eitffects. People trust bitcoin because it's pure, well understood and relatively stable. Yes, ETH may be better in some respects, but BTC didn't have to hard fork after the firs DAO. BTC hasn't been screwing up a move to POS for years.

BTC has tonnes of drawbacks, but the drawbacks are stable and known. And this confidence in Bitcoin stabilizes bitcoin, beleive in bitcoin because it has critical mass.

This may of course all be wrong, something may go wron tomorrow and completely hose the thing. But people generally don't think that.