As far as I know Apple never said anything about why they removed Airport extreme support. Transfer rates over 802.11g/n should be quite doable, especially for incremental backups. Those are usually not more than several megabytes.
I don't see any technical or practical reasons for the removal of support for Airport extreme. Besides the lack of support for symbolic hard links to directories. But all of technical things can be fixed with firmware upgrades.
My theory is that backup software requires extensive testing and Apple did not do enough testing with the Airport extreme so they removed the feature for now. I would be surprised if we don't see this option reappear before or during Macworld 2008.