I just don't think they have the time, and/or money to do both games in HD, so they're just going to focus on the extra modes and porting the arcade games. I'm sure they don't expect this release to sell as much as their more popular games like Deathsmiles or Futari, which is why we should be pretty thankful they're doing it at all.
Would you rather have MMP in HD as a standalone release or Pink Sweets and MMP together, but no HD for either? They probably mulled that question over and went with the latter to please their fans, and get the most bang for their buck.