Because clinical quality mRNA can be manufactured quickly enough to beat a pandemic (case in point: the two leading candidates). &, we’d worked with Moderna for years to understand how to make a good MERS vaccine & tested it extensively pre-clinically. https://www.nature.com/articles/s41586-020-2622-0 https://twitter.com/ethanjvaughan/status/1339444937272643585
Along with other scientific reasons why we were uncomfortable with traditional methods, like vaccine induced enhancement, which my boss summarizes here: https://science.sciencemag.org/content/368/6494/945.full
That said, I am optimistic that several different types of COVID vaccines will eventually get approved. If mRNA in all of it’s revolutionary glory isn’t your cup of tea... you will likely have an option that is shall you make the informed choice to be vaccinated.