Who's spinning the rear on takeoff and sliding the back all over in turns to want a wider rim and tire on the back for a small gain of extra rear traction (IF such small gain would even be had, which it WOULDN'T be had without a rim that matched)?
Why would you want to try and increase rear grip without corresponding front grip increase?
Who wants to ride a bike that would let the front slide and let go in a turn before the rear? A rear wheel slide is far easier for any rider to catch and manage than a front slide. The bike actually tries to catch a rear wheel slide on it's own better than a front slide. Front slides are often short and let go really fast in comparison. Even the best riders in the world racing MotoGP often get eaten alive by front slides, while being able purposely slide the rear in full control.
If the rear is too wide for the rim, due to deformation, you get a smaller contact patch down near the tire's edge at steep lean, where you really do want to have at least what you had before with the stock tire (because the factory hopefully tested and balanced the tires front to rear in size and behavior).
Practicality wise, you only have use for a wider rear tire if you are habitually spinning up the rear "to easily" on turn exits or on straight ahead acceleration. Then you give away turn in sharpness for the ability to use the throttle harder exiting turns.
Who needs to exit turns that hard on the street.
Or is it a bling thing, like Harley chrome? Give away performance for a certain look?
