People have always liked one color over another with the pokemon games. Most people liked fire over water, and therefore chose the origional red over blue. A lot of people buy FireRed for old times sake, so that explais some of it. Also, if you add all the pokemon in FR with the 200 pokemon obtainable in Hoenn, and then compare it with the combo of the pokemon in LG plus the Hoenn pokemon, FR comes out ahed. Basicly, you can catch more "LG" pokemon in Ruby and Sapphire, so people therefore tend to go for FireRed. Charizard has also been a favorite pokemon for a lot of people. Since charizard is on the box, most would rather go for it then Venasaur. Most people would also rather have a red game then a green one.
I, however, much prefer Green over Red (take a glance at my Screen Name). The chance to actualy get a Green GBA game just couldn't be passed up. Most people would be getting FR anyway, so if I get LG, I would have access to the pokemon that the majority wouldn't have access to (the ones unique to LG), and I could more likely find someone to trade with, to complete both of our pokedexes.
Anyway, those are my thoughts.