The simple answer to this is the number of walls. 2 vs 1. But what does it imply?

It’s not just that 2 walls keep you warmer than 1 wall in cold weather, double wall tents are known to keep you dry even better than single wall tents. This is because in a double wall tent, the fly keeps the rain and snow at bay while the inner tent expels the moist air so that it condenses on the fly (in cases ventilation doesn’t do the trick) which then drips to the ground instead of the user.

A single wall tent has to rely on one wall to do both i.e. deal with moisture from inside and outside. So the fabric has to be waterproof and breathable. Breathable fabrics may not be enough to battle condensation effectively so you have to rely on venting.

The upside to single wall tents is that they’re lighter and less bulky. Double wall tents have more parts and hence heavier and bulkier.