XP Caps are usually put in place by some developers to prevent boosting a game’s ranking system too fast (like in Reach and Halo Wars. Some games like Gears 2 and MW have no caps but just try to either have a scaling multiplier on some modes (Gears) or an action based XP system with a self-reset to progress (prestiging in MW) to prevent boosting to the max rank. Yet Halo Wars is over two years since release with a very low population, essentially a very niche (and IMHO dead) game.
The question is why does this game still have the cap at this point? What purpose does it serve for this game? Why not remove it?