Why? Well, ya gotta admit that a white squirrel is pretty easy for a predator to see, and I'm just guessing here, but I suspect it became food for a local hawk fairly quickly.
So, why is it white? The first and obvious answer is that it was an albino, which means that it lacked pigment in its skin and fur. But I am fairly sure that it is not an albino, as it does have some pigment, especially in the tail, and does not seem to have red/pink eyes (that is the dead ringer for an albino squirrel). Take a look.
So I am calling this a white morph and not an albino, and here is another look at it.