1) mRNA is fragile, relative to the pretty durable nature of many vaccines that have been optimized over the last 40 years to be idiot proofed. The temperature preserves the efficacy of the vaccine indefinitely, but the actual vaccine doses given to people is warmed to room temperature. At least one of the western states thaws the vaccine out for the day and then flown by plane to vaccination sites.
2) The real issue is dry ice is limited is because the #1 feedstock source of CO2 for dry ice is ethanol production. Because of lockdowns, fuel sales are down, hence ethanol production - that and the Trump keeps letting gasoline refiners waive ethanol requirements in US law. So blame Trump.
I am cool going without dry ice for a few months.