I heard a theory somewhere, (not sure where, coulda been CinnamonBunzuh or Poparena) that Jake never morphed Howler again because that would open him up to the memory network again, which could potentially mean being instantly deleted or possessed by Crayak within the rules of the cosmic game. Would you really want to expose your consciousness to direct contact with an incredibly powerful and evil extradimensional being? It is something Jake had nightmares about for months, maybe a year before #26 rolled around, so you can understand how he'd be kind of uncomfortable about getting close to that subject.
They never used the dino morphs because those were acquired during a sario rip, which is apparently a sort of time loop that resets everything in the universe except for your brain. Remember, Jake had a jaguar from #11, but he found out he couldn't morph it again at the end of the book because Jake ended up preventing the loop from happening, which meant that he never actually acquired it.