I can't believe no one has seen it yet...
Well, if they'd done a full animated recon it'd have been an instabuy, but since they didn't I'll wait till it goes under a tenner
They don't have the budget for it. Each DVD gets around £140,000, and it takes roughly £50,000 to animate an episode. The Aztecs SE would have included enhanced restoration of the main story, plus animating THREE episodes rather than the standard two would've blown the budget out completely.
It's highly unlikely the BBC will commit to animating anything beyond The Crusade, The Underwater Menace and The Moonbase, as they are the only stories that have two episodes left to animate...