Based on the fact that most 'Anime-based' games do not really fare as well as their programme counterparts, the deduced factor is the continued reused and worn-out story lines.
Generally, fresh programmes start out better than the evolved versions; Street Fighter Series did much better as a game rather than its television release. It is true vice-versa with the many examples expressed above.
But of course, fresh 'Anime' games, like Disgaea, with its original plots would do better than the 'Anime-based' rip-offs.
I do hope that the Disgaea Anime would not suffer in such an event, though...