Mewtwo was the ORIGINAL epic Pokemon. Lucario is lame, over-hyped, and undeserving.
there is one movie surrounding lucario which is only about mew so it in a sense only has him as a side character in the plot.
Mewtwo was one of the main focuses of the original generation of Pokemon, and has a cemented spot as one of the main powers in the series. Regardless of how many hundreds of new creatures they add to the series, all will remember the original badass.