You're forgetting the RPG titles. Some of the Mario Party games also had original characters in some boards (like the Koopa Kids/Mini Koopa in earlier titles). Yet, none of those ever made it back to the main Mario games. In fact, we got Bowser Jr. after the Mini Koopa, and Galaxy went and created the Luma even though Paper Mario already had the similar Star Kids.
It just doesn't seem like the teams handling the main series ever look at the additions made by the side games. It seems to be a strictly one way street even when they easily could have been used by the main series rather than original concepts/characters.