Having been participating on Stack Overflow a bit recently, I've noticed this:
This doesn't seem to exist on Ask Different:
Can it be added?
It seems only SO shows the rep. league.
Since SE has 115(!) Q&A sites all in all, I made a script (obviously :P) to look through each of them and check whether profile pages showed rep. league.
The result was that no other SE sites but SO showed it. To be extra sure, I also recorded total number of users for each site and number of users with actual RP change. And thus, I don't think it has anything to do with community size.
Just to give you an idea of community sizes on SE:
So maybe the SE devs are just trying it out on their largest site? Doesn't seem to be by mistake that it only shows on SO.