$500 million is about half of Barda's budget. BARDA's core mission involves developing and procuring medical countermeasures for public health and medical consequences of CBRN incidents, pandemic flu, and emerging infectious diseases ... the kinds of things where mRNA vaccines are likely needed.
If there’s advance warning of a potential disease outbreak, then the speed of mRNA is less relevant and conventional technology will likely be superior. If a threat is unexpected, a new program will be needed anyway. As such, I think prioritising established tech for vaccine development is sensible