There are no reasons. Whoever is in charge of the movie gets to choose what they want to do with it. If they choose to build it off Donner's film then so be it. If they choose to build it off Abrams draft then so be it. If they choose to build it off STAS then so be it. Singer has made his choice. It's ridiculous that you would think he should base it on what you prefer simply because you like it more.
LOL. Defending Singer's vision means defending his right to tell the story he wants to tell. If the movie is bad, I'll be more than happy to criticize it but I'll still defend his choices to tell the story he wanted to tell. It's out of respect of the artist and not simply dismissing his ideas simply because I do not agree with them. But I suppose such a concept escapes you?