since all these posts handle the same subject:
it's simple why it's him. 1st: he is (one of) te oldest captains in the Gotei 13. This gives him seniority and a position to be respected in. 2nd: he remains somehwat distant in his affairs (unless you are a girl, then he might be *ahem* a bit more approachable *ahem*). Even though he is a lazy bum, he know what shoudl happen and has never had real chaos rule his squad. It has always been neatly organized. This indicates that he has leadership skills (in particular delegating tasks). I did expect to see Ukitake with him on the HRM front of Seireitei managementfor the rest the chapter was fairly nonexistent in stuff happening.