The BF2 engine is awful, it's an abomination really. It's just not designed to be modded. You just have to hack things to try and get them to do what you want and hope it's possible. The dev team are HUGELY limited in what they can do. Frankly it's impressive how much people have got out of the BF1942/BF2 engines.
But while technologically speaking the engine is terrible, on non technical accounts it has some upsides. The basic game modes in BF2 are pretty much what's needed in FH2. Saving a huge amount of work. And you also have to remember that first, BF2 is a very popular game, and second, FH1 being based on BF1942 makes the engine a logical progression.