Sorry, but this has to be the STUPIDEST decision in the whole game (of which there are many). Likewise, limiting 'Infestation' mode to only Marines-->Aliens. AVP2 was fully customisable with its multiplayer modes and choice of species.
Another question: Why couldn't Domination be 3 way with all 3 species duelling for points, instead of just 2? It's unorthodox decisions like that that help make a game unique and popular. If people just wanted a bastardised UT mode, they'll play UT3.
So much wasted potential, and so many needless restrictions in this game. I'll never understand who made these decisions, because they're just limiting the fun factor and lasting appeal of a game that feels the same every time you play it.