Yes, but why that low?

Because it degrades at higher temperatures.

They do stability studies on these molecules, holding samples at different temperatures and measuring how much "is left" after a period of time.

This particular RNA sequence and carrier might be particularly unstable, so it require -80C if you want it to last longer than a few weeks.

Because RNA is not a stable molecule. At lower temperatures, it's less likely for a given bond to reach the Gibbs free energy of activation, thus preventing degradation.

In a biological context, the vast majority of RNA use falls within the 'central dogma' of DNA -> RNA -> Protein. It is merely an intermediate, albeit one with important and complex regulatory processes.

In RNA viruses and other contexts where stability is required, the RNA is stabilized by its environment, namely by associating with special proteins.