CRISPR is basically find-and-replace with DNA. DNA expression is weird and complicated, and some bits do different things depending on the environmental context. There's a lot of potential for things to go wrong.

mRNA vaccines are injecting simple, engineered mRNA. This mRNA is expressed 100% of the time; what it does is very simple. I'd be confident using a new mRNA vaccine on day 1, so long as they're certain they picked the right protein, but I wouldn't be confident using a CRISPR treatment without a full medical trial.