Speed and liability, and to a lesser extent, MPG.
RAM had to get tires specially designed for the TRX, whose speed limiter of 118 mph is based on the tires.
There are few, if any, other 35" offroad tires out there with a speed rating over 100 mph.
Now consider 37s - what would be the point in having a Raptor R that is rated for 90 mph top speed or something ridiculous?
Sure, you can put 37s on a TRX, but at that point, the liability is transferred off of RAM and onto the consumer.
Not to mention the reduced handling, higher center of gravity, etc. with 37s. And we all know how eco-conscious Ford is now, so while the MPG loss with 37s won't be much, it will matter.
So no, I don't see Ford allowing 37s with the R.