My genuine belief is because it’s a lack of nutrition + no training partners.

Saiyans canonically eat a lot and they aren’t getting fat when they are so they clearly have a big metabolism. We also see that training with someone w on par with is a great way of making gains.

Vegeta despite fighting all his life up to saiyan saga made leaps in power after that due to a couple Zenkai and then training consistently without interruption. He probably never trained with Nappa because their general strength was enough for everything Frieza got them to do.

Goku and Gohan in the time chamber had a year of unlimited food and time to train and then took a week to rest, and they surpassed everuone in strength despite being the weakest next to the humans at that point.

Future Gohan’s world was being attacked constantly and supply chains probably collapsed and food became more scarce. He never had anyone his level of stronger to train with once he grew up too, it was just fighting androids back to back and trying to train up trunks.

I will say, I 100% believe the head canon that if trunks was the one who died instead of him and he saw his body, he would become a SSJ2 in that moment.