DNA uses base pairs [A,T] and [G,C], this code is for a piece of DNA,. if you keep a DNA sequence in vials for later use, that is much more stable and easier to manipulate, and repair when corrupted.

normally RNA in vivo is complexed with protiens that prevent RNA from folding, and annealing into structure that is not compatible with translation to protien. In the vaccine this isnt happening, this is why RNA is hard to work with and the vaccine must be kept so cold.

This is not to say that DNA is simple to work with, but it solves problems if you dont need direct access to RNA.