I think a lot of it is that clinical trials are expensive and drug manufacturers generally try to maximize odds of success. COVID was unique because they could

I think a lot of it is that clinical trials are expensive and drug manufacturers generally try to maximize odds of success. COVID was unique because they could get a ton of money to develop and test a vaccine that they knew there would be a ton of demand for if they were early to market with something that worked. now that they've seen how much easier and cheaper mRNA vaccines can be to develop, it's much easier to decide to start a trial.