What’s the real reason that Gohan became weaker? Why did he stop getting more powerful and succeed his father? At the end of Dragonball Z Gohan was arguably the strongest unfused character but what happened to him
The simple reason is that he didn’t have a rival. Gohan didn’t have someone to push him to continue becoming stronger.
But why was he so strong in the first place? How come he could achieve ssj2 before anyone else? What made him so special to defeat Cell?
He had something to strive for back then, he had a purpose. He had to protect those precious to him and that gave him his strength.