Why are white tigers white

I've always wondered why they're white. I think they're beautiful, but I know that everything in nature has certain traits to survive in their environment. So what's the "reason" for them being white, and if there is no "reason" then what is it that makes them white instead of orange, and why are they still considered the same subspecies as normal colored tigers? What is it that makes them white? Because I doubt they're albino or something...
Thanks in advance :)

White tigers are apparently very rare in the wild, but more common in captivity. This makes it quite likely that they are a consequence of in-breeding. Of course, as they are a great attraction it is possible that the zoo owners then deliberately breed more by using "white" tigers as parents.

Mutation

Genetic mutations.