Because it's kinda difficult to take the show as it is and insert an entirely new character while keeping the essense of the scenarios and results the same as Canon... Ne, Keroko?
But on the other hand, I've seen some already. Like the StrikerS Final Battle inserts. You also have links of them as far as I can remember. But that should be rather spoilerish for Fenris to watch now.
Oh, and BTW. Mewants that AU fic too! >:3
(No pressure, though.;3)