Well firstly they shouldn’t be allowed simply for politics and practicalities.
The Six Nations is just fine the way it is and there’s no room for another country/team to be included. The rugby season is already too congested, and to try to crowbar in another match to the Championship would cause problems. This means one team (obviously Italy) would have to drop down into the Rugby Europe International Championships. Georgia has won 10 of the last 11 of these. None of the 6 Nations teams would get anything out of playing against Romania, Germany etc. So it’s in their interests to avoid dropping down.
Secondly, and perhaps more importantly, money. Georgia is not wealthy, nor is it a famously pleasant holiday destination, its probably lovely to the natives but seriously who’s going to be hopping on the Ryanair website looking for deals to Tiblisi. At the very least when teams play Italy away, travelling supporters can spend a weekend sightseeing in Rome. I doubt they’d do the same for Tbilisi. And quite frankly, the TV revenues would be a lot lower if Italy were dropped for Georgia.
The sad part is, Georgia quite likely should be in the 6 Nations. Unfortunately it probably won’t happen and I for one would agree with this .