If you read up on various cars with belt/chain drive CVTs, you'll notice that they all have reltively low torque ratings. It takes a lot of force on the belt to keep it from slipping. Not sure what the term is, but you couldn't have a belt with the groove/ridge thingy's that engage a cog with a CRT, which means it's even more likely to slip. This adds up to two things, one it'd have to be way overbuilt, and two, much of the gains you get from having a CRT would be cancelled out by higher friction.