Covid poured accelerant on MERS vaccine research. Suddenly, there was a disease with lots of cases with lots of people willing to sign up for trials. In addition, there was a lot of money sloshing around so companies were willing to take the financial risk to combine Phase II and Phase III trials (to be fair--a Covid vaccine passing a Phase II trial was always going to pass a Phase III trial until we had many different vaccines). And, please do remember, nobody knew if mRNA for Covid was actually going to work.
Other mRNA vaccines are in the pipe. However, they have to go through trials with a lot less funding and a lot less money at the end. Rabies(!) got a Phase I trial--but it's hard to get to Phase II since rabies is so rare. mRNA flu vaccines are in Phase III. mRNA HIV vaccines are in Phase I and may be in Phase II by now.
What you see is that if we drop the equivalent of the Manhattan Project at producing a vaccine, we can pretty much produce a vaccine for a virus in a hurry. However, if we're not willing to drop that kind of cash, progress is slow.
If you want more vaccines, tell some of these multi-jillionaires to quit fucking around with virtual crap like Bitcoin and Twitter and start dropping some non-virtual gigabucks on vaccine companies that are trying to produce real products.
