In short, because RoF is a different engine built with different things in mind and having different priorities and goals.
In some other forum, someone might be talking about how you can't even load 200-300 units in a RoF mission because it crashes and posting that "they managed to make CoD load missions with 15000 objects despite all the other bugs, why can't RoF which is more polished do the same?"
Mind you that's not a number i pulled out of thin air. The CoD map running on the Sundicate server has 15000 objects and 5000 of those are flak guns.
