Re: Why didn't Kirk just contact Sulu?
As for the original question, maybe Excelsior ferried delegates to the conference. Which would be how Sulu knew where the conference was...
But in Flashback, Excelsior was trying to rescue Kirk and McCoy while they were held captive by the Klingons. Not much time for them to be ferrying diplomats.
"I was never a Star Trek fan." J.J. Abrams