Because that is the calcium and vitamins that works best with that diet. Bml has been around for years and is one of the most widely used diets out there, and one of few diets that the creator isn't making any profit from and it has been through testing.
Personally, I wouldn't feed mine anything from exotic nutrition the only thing I'd buy from them is cages.
You won't know if she'll eat it unless you make it for her. There is no guarantee a glider will eat the diet the human chooses, all you can do is try it for a couple weeks and see how she does.
How old is she?