This is strictly from my (often faulty) memory, but I believe that the -er and -or (instead of -our) are from the spelling reforms of Noah Webster, at the beginning of the nineteenth century, in his attempt to regularize (!) and Americanize spelling. The standard American spelling is theater, but it is very common for a building or an organization to use theatre, based, I think, on the notion that it is more arty. The small town that I live in actually has two theaters that use the -re spelling.