There is nothing particularly special about the size, car makers just haven't spec'd a tire in that size before. Why is that? Well, there are many many trade offs involved. The most obvious being that for a fixed tire diameter, larger wheel diameters reduce the height of the sidewall which compromises ride comfort and increases the risk of wheel and tire damage but can also improve steering feel and handling. This is why most vehicles running large diameter wheels tend to also have low profile tires - there's only so much space available inside the wheel well. Off road vehicles can benefit from larger diameter tires and are also more likely to be designed to have room for mounting larger tires. But in this case it is especially desirable to have a much taller sidewall thus they tend to avoid large diameter rims. Then there are grip and contact patch considerations associated with a high torque power train. A Dodge Hellcat might opt for wider tires of smaller diameter (wider contact patch) because mpg is already abysmal and a very large diameter tire doesn't match the proportions of the vehicle. An electric SUV trying to optimize for range however might optimize for taller, narrower tires (longer contact patch) to minimize aero drag associated with increased tire width. The larger tire diameter also benefits offroad. Rivian also needs to span the gamut from street to off-road and most manufacturers try to minimize the difference in tire diameter across option packages. Rivian has also chosen rather sizable brakes which limits how small a wheel can be used with offroad configurations. It also wouldn't surprise me if Rivian was trying to minimize the variation in range across their tire/wheel packages and this could have resulted in their most efficient setup being the middle size option. But I'm just rambling. The point is, there are many many trade offs involved when a manufacturer selects a tire and wheel size.@electruckWith that said, are you able to shed some light on why the 275/55R21 size hasn't been produced previously? What is so novel about that size, including its advantages/disadvantages, and why do you think Rivian worked with a manufacturer (Pirelli in this case) to offer that size specifically?
Bingo... if the market for this tire size is limited exclusively to a subset of Rivian vehicles which are going to be produced in low volumes for the foreseeable future, it's probably not going to be a compelling market for other manufacturers to jump into.
There are actually a few choices in 275/50R21, including snow and summer options, and it would be ~1.1" smaller (~3.3%).The MDX will have a 275 on a 21" wheel but it will be a 275/40r21 which is about 3.2" smaller in diameter than the Rivian's 275/55r21.