Ryoma has both Nanjiroh and Tezuka to help him grow. Who does Tezuka have? We hardly see Tezuka play. When we do, we hardly see him challenged. (Moreso now in the manga, though it seemed odd that he was challenged in that last match.)
We know he and Inui have played a fair amount, but the last match was the only time that Tezuka really had to use his full ability. That's not actually stated, but it's obvious from the reactions of both Inui and the spectators.
How can Tezuka improve as a player if he's not playing someone better than himself? Or at least someone roughly equal. And while I'm not opposed to the idea that he's been playing Yukimura every weekend for the last three years, there's no evidence that Tezuka has any sort of training partner or mentor or coach outside of school.
Is that maybe his problem? His ability is very strong, but relatively static, because he doesn't have anything to push against.
We don't know anything about Tezuka's practice and training regimen. I'm sure we've been told it's copious and strict, but that's it. We've seen him using a ball machine, but that's no substitute for actual play.
Maybe that's one reason to raise Ryoma up. Once Ryoma can defeat Tezuka, Tezuka will have someone he can play with.