Gamefreak wanted to showcase the new Generation. They used Munchlax to show off the new games. Why they did it exactly, I don't know for sure. But I'm pretty sure they used it to give a little introduction.Hope I helped!
>Munchlax fact Bulbapedia (It was the first Generation IV Pokémon to be revealed to the public, in May 2004.)
Well I guess you first saw Munchlax in the destiny deoxys movie. The Movie was released
July 17, 2004 (Japan)
And Munchlax was revealed in May 2004
Hope I helped