But the thing that bothers me we keep seeing him in full mammoth form. While fighting some of the fastest and most mobile fighters we've seen. Makes no damn sense to assume that bulky slow form, or to be shown in that form at all during this fight. Did Oda not want to bother drawing it, knowing he will get off paneled anyway. It's still sloppy as far as I'm concerned.
Gifters: Lord Jacku, we're being overwhelmed! You have to use your trump card, please use that transformation!
Jack: Cough …I.... I can't! I will never use that form, even if it kills me!
Gifters: Lord Jackuuuu?!??!
Jack: We're not the Hybrid Mode Pirates! We are….
! THE BEAST PIRATES!
!
Seriously though, I think it's obvious why Oda is holding those forms back. But even if it weren't, the Beast Pirates making more use of their pure animal forms makes a lot of sense to me, simply because they're the first type of Zoans with useful "base animals" (outside of the Mythicals).
Now Jack was tasked to stop an army of Sulong Minks, so he needed durability (attacks would come from all sides, evasion is not an option in a dogpile scenario) plus the necessary range / area of effect to stop them from reaching Kaido.
And it's not like this tactic didn't work, at least when we look at the panels where one trunk-swing alone wiped out an entire Minkssquad.
Finally, we don't know how Jack's hybrid mode will enhance him. He would be obviously more agile compared to an elephant, but he's still a fat Mammoth Zoan used by a fat half-fishman dude.
I wouldn't expect him to zap around like Lucci or Jabra, who had agile base animals + full Rokushiki mastery.