Contracts end. It's a shame, but that's life. At least there'll be space for a full DoA cast of favourites, rather than there being the risk of having to drop anyone major.
Plus, DLC, right? Gotta have those monies pouring in later.
The whole expiring license thing is one of the reasons I personally dislike guest characters as a general rule. Also never really liked Mai and nao all that much to begin with so there's that too. I'll be kinda sad to see the vf characters go though.