The big reason is that recovery is not so easy. If you lose your wallet and have to recover with your seed phrase, the coins might be there and locked to your wallet, but hard to figure out that it's there. You need to back up the output descriptor somewhere, which adds another step.

I think this will become standard with TapRoot. Wallet providers will be a lot more willing to experiment with this stuff because coding it isn't so nerve-wracking. I know for myself, it's something I want to play with because it's so much easier to add as a scriptspend condition.