Developed countries don't have infrastructure for -70C distribution. The US is going to struggle to actually distribute the Pfizer vaccine. It's not like your local CVS has a freezer that goes down to -70C. Dry ice is possible but there's a shortage of that, too. Nobody has ever distributed a vaccine that needs to be stored at -70C.
The Moderna one is much easier, you just need a normal freezer. Which is fine for developed countries, will still be a challenge elsewhere (but one they're used to, it's not the first vaccine that needs to be kept cold)
There is no shortage of dry ice. The input (CO2) is a byproduct of industrial processes (ethanol in particular) and so the market is completely glutted. It is also used for a number of industrial process so there is plenty of infrastructure to produce it in any city.
The Moderna one is much easier, you just need a normal freezer. Which is fine for developed countries, will still be a challenge elsewhere (but one they're used to, it's not the first vaccine that needs to be kept cold)