It's not like Chen or ch'Tharvasse are bad candidates. Far from it, they'd be great candidates any year. Chen is apparently brilliant at everything she does, from captaincy to the logistics of shipyard ops. She would have new ideas for Starfleet almost certainly. ch'Tarvasse is a very old hat and has managed much of the duties the Commander has, in addition to picking up both Kahurangi's and Sousa's political legacies. I imagine, for example, that he's seen the difference in attitude that Kahurangi and Sousa took to the FDS. Sulu, for all his popularity, is also brilliant. And the sheer number of people in all levels of Starfleet and the Federation at large that he can draw upon gives him an incredible edge in intel, and in getting non-Starfleet capabilities to help Starfleet and Federation purposes. If Sousa was the ultimate politician, Sulu is probably the ultimate diplomat. Well, until Uhura comes along.