Offkai helped Fishman meet a bunch of goals for conventions appearances that Anime Expo wouldn't:
1) He got to showcase his talents with concert performances, meet and greets, panels, and so forth. None of these would have happened had they concentrated on Anime Expo, he would have gotten a small booth and no extras instead.
2) Got to showcase Wonder AI
3) Made good money off merch sales
4) Extremely targeted advertising of talents to people already predisposed to watching vtubers
Sakana chose to be a big fish in a small pond, rather than a small fish at a big pond. Maybe one day Phase Connect will have a large booth at Anime Expo and be an important player there, but what we have right now is pretty good for a small vtubing corpo with limited resources. I think Fishman made the right choice.