There is no gameplay based reason against those kind of a faction.
Fun and selling point. People are more excited about playing with big/iconic toys rather than "low tier" non medium/heavy vehicle based armies or early war tools. Combined that with limited human resources/capital and risk analysis and you have your reasons.
I'm not against implementing any type of new content or faction, if it's in the right contest (say you implement Japs vs USF but that's a fix match up on specific maps) but Relic probably has a chart on which they have gauge interest of what people might prefer, and i bet "lesser" countries doesn't have enough public in order to invest time and money on them.
"Why this mega corporation working on a niche genre, doesn't produce an even more niche faction/theme based game?"
But i see why Relic wont try such a scenario.