Because wallets don't support it, because there is no economic incentive to switch.

MuSig is a strong reason to switch, for multisig use-cases, but it was not standardized or available in libsecp for 2 of the 3 years (something like that).

Taproot adoption was always fundamentally dependent on its adoption within lightning (at least, that's what I was saying 3 years ago and I *think* I've been proven right). When that happens I believe adoption even for non-lightning use cases will increase substantially.

But that apparently is going to take more time.