The Nanosuit 2 is probably more powerful then any vehicle this game has to offer. Not to mention the urban setting simply does not work for vehicles, especially airborne ones. And besides perhaps the tanks, Crysis has had pretty awful vehicles mechanics. A helicopter can point 90 degrees and not lose a foot of altitude. A VTOL can turn on a dime and go backwards, even though the engines don't maneuver that direction. Humvees are slow and hard to turn. Watercraft are essentially useless. And pretty much every ground vehicle in the original could be rolled over by a small pebble.
Crysis 2 doesn't really need vehicles if you think about. All it needed are the driveable APC's/Jeeps the game already has. Besides, PC modders can make a map with them if they feel like it.