Was it ever explained why...

Shunsui is so unimpressive compared to how old he is? Had to get help with Starrk, Ukitake was equal to him and he was sick all the time, Adult Toshiro, Kenpachi, even a goddamn Liteunant in Anime Renji is stronger than him. Plus he got beaten by Robert

You would expect the Captain Commander of the Gotei 13 to be stronger. Especially since his Bankai is incredibly overrated. All you have to do is have more Reitsu than him and Stage 1 and 2 become useless.