For as far as I know it is a matter of weight distribution. The rear end of a 2WD buggy and truck is very slippery. With the added weight from the motor and battery it improves grip on the rear.
I had my 4WD front belt broken during a race some time ago, but it did act very weird. In no way as good as a 2WD buggy. Too less grip on the rear side made me go 6 seconds slower than I normally did with 4WD. Also the jumping was a little different. Of course you can do something with the setup, but it would never go as quick as a 2WD.
Also converting your 4WD to 2WD on purpose is something I wouldn't advice you to do. These cars are simply not meant for doing this.