Manufacturing capacity.
mRNA method takes advantage of the existing manufacturing capacity of 8,000,000,000 homo sapiens.
Whole dead virus is of course the traditional method (that, and weakened live virus, such as oral polio.)
For one, manufacturing in vats is more efficient than drawing blood or other samples from thousands of monkeys, chickens, eggs, etc.
For another, those chickens turned out to have other pathogens that cause cancer, monkeys carried HIV, etc.
If mRNA could be successfully and safely used, it would be a miracle treatment. It does have many years use in veterinary vaccination.
But with massive deployment, all the differences in patients and administration expose issues. Whether side effects or ineffectiveness against variants.
Maybe against other pathogens which don't mutate rapidly if would be more effective.
Maybe, but it took a couple years to be made to measure for new variant. After starting initial version within 3 days of getting the genetic code over the phone.