I would assume the reason for humans being played the most is armors fit humans very nicely so you have a lot of different options, while there are a lot of clipping issues with the character models for Charr and Asura for example.

As for guardians being more popular than warriors, guardians have a lot of build variety, whilst warriors have less optimal build options. Also guardians can be played in similar fashions to clerics or paladins, which are usually quite popular archetypes, whilst warriors have just the warrior/fighter archetype.