Why Is Master Roshi Become Weak?
He doesn’t train.
As far as we know, for hundreds of years, between the fall of King Piccolo and the events we see in Dragonball, Master Roshi was the strongest man on Earth. That’s nothing to sneeze at. The only people who were able to surpass him were his own students (with perhaps the exception of Tien, who later studied under him as well, to a small degree).
However, during Dragonball, we never see Roshi training. He teaches, so he participated in the training of his students a bit, but that hardly pushed him to his limits. He fights on occasion, like in the battle against King Piccolo, and Golden Frieza, but for the most part, he sits on his island looking at magazines full of naked women and watching women’s aerobics videos. He doesn’t spend all hours of the day working out and practicing energy blasts like we see Krillin and the rest of his students doing. He also never trained with Kami, King Kai, or anyone else, likely because they didn’t see the potential in him that they saw in his students, precisely because he doesn’t have the drive they do.
Since he’s immortal, it’s unlikely his age is hindering him as much as some might think. If he applied himself, he could likely grow as strong as any of the other human characters. He, presumably, just doesn’t feel the need, now that another generation has taken over.