I assume it had something to do with the fact that it would require a lot more resources from peoples pc's.
I dont know if the devs ever planned to make trees solid, but the games horrendous first days basically put them in damage control, rather than development mode. They definitely didnt want to start adding more resource intensive features while some people couldnt even play it as it was.
Nowadays, it would probably go down fairly well on high end pc's (dont quote me), but theyve stopped developing CloD anyway.
Im guessing, if they decide to implement solid trees at all, it will be in the sequel.
Rise of Flight is just an A+ game. I dont know how they do it, but they just seem to be able to put whatever they want in their game!