There's not much visually distinct about Baby Bear that distinguishes him as a specific Sesame Street Muppet when it comes to merch. Part of the "core cast" thing, to me, is recognizing that "these Muppets sell the most toys, ego show them more often than not."
Yes, there is not much Herry or Mumford merchandise, but at least if I saw a Herry Monster doll in a store, I'd know who it is. If a Baby Bear toy was moved from the Sesame section to a generic batch of animal dolls, he'd fit right in.