MonoGame is meant to be an OSS XNA replacement. It's very good for indies with good programming skills. Fez and Bastion were implemented using XNA/MonoGame. If one is more of a game-designer/scripter or is business-oriented then Unity3d is also a good choice.
Completely anecdotal, but I would bet it was due to difficulty getting returns from the program. DirectX itself helps sell licenses of Windows, but XNA not so much due to only impacting indie games.