Although it is based on usage, if a Pokemon is extremely powerful, or "good" in another way, people will probably use it, which gives the same result. However, when the Pokemon haven't been avaliable for very long, no one is really sure about this, and there's more random stuff, which decreases over time. I have no clue what happened to Celebi, but lack of Mew and Latias may be due to the fact that many people still consider them Uber, and just don't want to use them