The why not boils down to MEV that's difficult to predict how it will behave. Since drive chain anchor transactions can be very high value, it could become routine that optimal miner behavior is to attempt a re-org to capture more blocks (as an example).

OP_CTV, OP_APO, OP_CSFS (I am way out of my depth here) are more interesting covenant options imo. @4moonsettler and @polyd_ on X are sharing a ton of educational content about how those things could work. Recommend reading up on it.. it's pretty mind blowing.