Why was so much content taken out of Halo: Reach?

In Halo: Reach, they took out the Hornet, the Brute Chopper, the Brute Prowler, as well as many more vehicles, as well as weapons such as spike grenades, the brute shot, the battle rifle, and the SMG. One might argue that all the removed weapons got a counter-part in Reach, but it seems like there wouldn't be a reason not to include them in the Forge. Also, how come the civilian vehicles and the option to sit in the Falcon penger seat aren't available in Multiplayer?