Well I think nobody will come out and say we have a very faction balanced roster. There certainly could be more toons/ships in certain factions. There are some factions, when CG created them they might have had plans to grow them but then over time deviated from their plan and other factions become more popular. Also, this is a Star Wars universe brand game, the developers can't just freely invent new characters as they wish. They have to be inline with the canon. And last but not least, CG also has to follow the way Disney develops the Star Wars franchise, if they want to stay commercially relevant.
On the other hand, factions are overrated. Personally, I like plug&play characters/ships, which work well across factions. That gives much more room for theory-crafting. This is just another aspect.