was created by Team Rocket scientist Blaine, who added his own human DNA to Mewtwo to finish its creation when Mew's cells weren't enough.
Mewtwo wasn't created from a single cell, it was created from several, according to Bulbapedia, although they weren't enough, so Blaine added his own cells, hence Mewtwo's human-like form. If they were to make it from one cell, then it would have been very hard to create, seeing that a single cell cannot be seen by the naked eye, and would only provide the "coding" for that particular cell, and there are several types within one life form(except for unicellular organisms). That is in real life though, and in Pokémon anything can happen.
There really is no answer to "why". If you mean how, then as I already said they didn't make it from one cell. Although if they did then it is because they could, and that is all they could get of Mew's cells.