Another thing is, the WB is a separate entity from WB Television Production, and the various arms of WB tend to sometimes keep each other at arm's length. This is particularly an issue in the case of B5-related stuff, because Babylon 5 was once one of the shows on Prime Time Entertainment Network (PTEN). PTEN was a consortium of TV stations and WB, sort of a proto-network. When the WB network got started, it viewed PTEN as something of a competitor, so even though I'm sure WB Television wouldn't mind selling the show to the WB, if the price were right, I doubt the WB would want to buy it.
I'm not disagreeing that, ideally, WB Television would love to sell the show to a different network. But it's going to have a harder and harder time doing that, because so many networks are looking to become production companies themselves, and own the shows they make. That's part of what caused the holdup with Rangers, and it's part of the reason that WB and UPN got started -- self-defense on the part of the production companies.
Another issue is, really, I doubt that Rangers would be such a great fit for the WB. Although the younger cast does make it a closer fit than, say, Crusade or the original B5 would have been.