I would say that Mew follows Mewtwo in the pokedex because, while there were rumors and speculation of Mew, there was never enough concrete evidence to put it in the pokedex. Also, if memory serves, Mewtwo was created using Mew's DNA, which means that people could have thought that Mew was actually extinct, and extinct pokemon aren't put in the dex unless they can be brought back (like the fossils). So, while there was proof that Mewtwo existed, there wasn't proof of a live Mew until after Mewtwo was recorded, so Mew follows Mewtwo in the pokedex.