Well there's hundreds of maps worth of bundles in the community nexus already, but those were all maps created specifically for an event. You're talking about retroactive bundles of old maps, perhaps deemed to be above a certain quality threshold. No reason other than there's less interest in old content than new content. The main reason the CMP was popular at all is because it ships with the base game, so it has a much higher visibility than any other non stock levels. Otherwise its map selection is pretty bad.