Well, the current wireless stack on FreeBSD is a bit younger than wired networking, which has been there "forever" (it has been improved in many ways during this time).
Perhaps the developers who wrote the new wireless stack found this domain (wireless networking) sufficiently complex that they found out that the current level of abstraction / separation was needed to get a clean and well working implementation?
If you are interested enough, the mailing list archives for that period is available, perhaps you could find some interesting threads on this subject there.