[quote=@Ellion]
[@ProPro][@Halvtand] wait! Come back, I want to discuss role taking philosophies too!
[/quote]

Just jump in dude ;)

My next theory is that the non-bender in entirely open (like ProPro stated) and you're pretty much free to do whatever you want with it. The benders are sort of limited. It's not just which element or subelement you can bend, it kind of limits a lot of things. Certain elements draw upon certain things, breath for fire, balance/rooting for earth... and so this will be an important part of the character. The original cast is also a factor, because they are the source of these "important factors", but you don't want to clone them. So earthbenders need to be forceful, airbenders should be influenced by their evasive nature...
The fighting style is another factor. You can't just get into this and think "I wanna be a bender" because the different styles are so very different. An airbender cannot fight like an earthbender, it wouldn't work.

In short. To want an elemental bender, you kind of need to like the whole concept of that element, the personality, visual style, everything. And depending on what kind of a person you are, you're going to like one or two elements more than the other. 
At the same time, there are the non-benders. Completely unclaimed. Sure, there are some factions like the Kyoshi warriors and the pirates, but the aren't very well defined in the show, so you have a certain amount of room to improvise.
That's why there are less people on each element, but more on the non-benders. A better way to compare would be to look at (all) benders vs non-benders, in which case the split is pretty even.