The real answer is that we brought them along when we went to space. A better question is, why can tardigrades survive in space if it isn’t their natural habitat?

Cross-tolerance is a way of explaining why organisms may be able to thrive in environments that they have no reason to be adapted for. Take the example above, without us tardigrades would never be in space, so why would they evolve to survive being in space? The answer is that they didn’t, on earth tardigrades can survive in some of the toughest environments and the same things that allow them to do this may allow them to survive other stresses like being in space.