The first WR450 had the same fork that the YZ450 had, a 46mm KYB
single chamber unit similar to the earlier forks going back to '99 or so. The '04 YZ got a new 48mm SC fork, but the '04 WR kept the 46 for another year.
Then, in '05, when the YZ stepped up to the twin chamber AOSS fork, the WR inherited the 48mm single chamber from the '04. The WR has had that same fork ever since, and pardon me, but, it's inexcusable, IMO.
They aren't "leftovers", either. Holdovers, but not leftovers. The way the Japanese work inventory control, the forks don't exist until right before they get installed, and there is rarely ever any excess stock of anything. It's a shame, if you ask me, and other than the fact that it allows the bike to be made for less, I have no idea why they do it.
But, the 48mm Single Chamber is not a bad fork, and you can make it work pretty well, especially if you send it someplace like Enzo and have it gone over. The problem with that is that it's about the same money as you could spend on a used set of the new SSS forks and a pair
of springs, and that setup is better out of the box than most reworked SC's are.