How the hell did no airbender survive the genocide. The are fucking nomads. Some would not have been at the temples, while others should have been able to escape! It is literally impossible for Aang to be "THE LAST AIRBENDER". Some one please explain!

Story reasons. thats how.

6043359
They were probably caught off guard at a time when they were all either at or going to a Temple, and those few that remained were hunted down on the off chance one would be the Avatar.

6043359
It was stated in the show, by Aang no less, that it was a surprise attack and later by... I think it was Azula... that the Fire Nation was using the power of Sozan's Comet to destroy the Air Nomads.

With that in consideration, I rather doubt the Air Benders ever knew what hit them before they were killed.

6043359
Through the use of a ruthlessly efficient army that tracked down every single one of them and killed them.

It's sort of hard to hide when you have noticeable tattoos on your body and a particular manner of speech among other things that scream " Air Nomad!!"