it's called, bad plot. He didn't need to train to reach his potential, he was fighting against two androids far superior to goku in power for many years. If DBZ followed any logic, he woulda been stronger than the androids eventually just based on Zenkai boosts.
It was Goku who suggested there was a level above super saiyan. Future Gohan thought super saiyan was the pinnacle. So he didn't even attempt it. By the looks of it he didn't even continue training on his own thinking super saiyan was the extent of his potential.
TRUTH is future GOHAN had a thing for Bulma. He became a step-dad to TRUNKS. Gohan then trained Trunks so Trunks can go back in time to change the feeling he has for Bulma ❤
Rage boosts can only go so far. He never mastered SSJ1 like he did in the time chamber with Goku. Goku taught him how to master it. That's why they decided to stay super saiyan 24/7 leading up to the Cell games. They made SSJ1 Gohan's new base, so when the rage boost kicked in, it sent him to another level. Future Gohan didn't do that.
I believe master roshi had a hand in gohan training and discipline to a degree, we can say what you want, but gohan had a happy, go lucky demeanor when he wasn’t fighting and I don’t think he was putting on a front simply for trunks, that was genuine joy in a bleak environment much how Goku was during turtle training
The hopeless of his situation got to him, no matter how angry he got he still wasn’t strong enough and it just made him sloppy, and after constant failure he became jaded and cut himself off from his emotions just so he could keep fighting a losing battle.
Tien and old vegeta always trains alone without much improvement piccolo was the first to branch out from this mindset and started training with others then vegeta realized how much he improved with future trunks and kids trunks wasn’t the age to push him but he achieved super sayian two anyway
He trained himself to be a cool, calm, efficient warrior, surviving an unspecified number of battles with the Androids directly. This mindset is why he couldn't lose his shit anymore and tap into his potential. He had to lose control. His success was his demise, but Trunks was that success's legacy.
The reason is very simple and it's not got anything to do with him not having training to learn, as he achieved Super Saiyan after seeing those he loved die in front of him by the Cyborgs.
In the manga that the The History of Trunks special is based upon (as it isn't one of the theatrical films & is based upon manga canon material), Future Trunks already has access to Super Saiyan while Future Gohan is still alive, however he lacks Gohan's experience & since Future Trunks' Super Saiyan form is unmastered, he is more prone to being overcome by his emotions clouding his judgment (Future Gohan by comparison has been fighting the Androids for years & has likely learned how to keep his emotions in check similar to Goku during the latter parts of his battle with Frieza (despite being a Super Saiyan, Goku was willing to let Frieza live once he started running out of steam due to the strain of his 100% Final Form, and showed Frieza mercy when he begged for it after injuring himself with his own technique, & even the Angry Kamehameha didn't finish Frieza off as Goku admitted to Future Trunks that he had been too soft on Frieza)! Future Gohan was able to keep his instincts in check, though his greatest weakness was he he had the same protectivness of Future Trunks, that Future Piccolo had for Future Gohan, thus put his survival over his own, believing he had the potential to surpass him due to Future Trunks being the son of Vegeta (in the manga Future Trunks' access to Super Saiyan not unlike like his main timeline counterpart) likely made Future Gohan see Future Trunks as a prodigy like Vegeta! Future Gohan like his main timeline counterpart before the Cell Games also sincerely underestimated his own latent potential (presumably due to Guru unlocking it on Namek) & never suspecting there were levels beyond Super Saiyan (thus focused on simply getting stronger through training, rather than trying to push beyond the Super Saiyan barrier to become an Ascended Super Saiyan (aka Super Saiyan 2 which apparently requires mastering the base Super Saiyan form to achieve Super Saiyan Full Power which gets rid of the restless feeling, conserves ki, & effectively treats the form as no different from their base form, thus they can stay transformed even while at rest!
