Because when someone builds a hugely popular exotic system in the future, because it is one (1) cent cheaper, you'd end up with code that has to check to see if it's running on such a system.
This doesn't make any sense for multiple reasons, but especially because you wouldn't be checking anything in the first place. A big endian system would would reorder bytes and a little endian system would just use it directly from memory without another copy or reordering anything.