By "in use" you mean never progressed beyond the trial stage? Your article literally says in the opening paragraph, "why did it take until the global COVID-19 pandemic of 2020 for the first mRNA vaccine to be brought to market?"
The COVID vaccines are so poorly understood that the manufacturers claimed 95% efficacy against infection, a claim which evaporated within months as that number dropped through the floor. They have never even tried to explain why their trial results were so badly wrong, meaning they don't understand their own products. A product so new even its own makers can't explain its behavior is at best experimental, and that's being generous in the extreme. A more accurate assessment would be that COVID vaccination was a mandatory medical experiment forced on vast sections of humanity often against people's will, in which the people doing the forcing then turned a blind eye to any negative effects.
The COVID vaccines are so poorly understood that the manufacturers claimed 95% efficacy against infection, a claim which evaporated within months as that number dropped through the floor. They have never even tried to explain why their trial results were so badly wrong, meaning they don't understand their own products. A product so new even its own makers can't explain its behavior is at best experimental, and that's being generous in the extreme. A more accurate assessment would be that COVID vaccination was a mandatory medical experiment forced on vast sections of humanity often against people's will, in which the people doing the forcing then turned a blind eye to any negative effects.