Possibly because it's easy to get to, being within walking distance of the CBD. Island, Evans and Lyall bays shuld be reasonably easy to get to by bus, but some of the other popular bays (Worser, Scorching, Owhiro) are tricky at the weekend for those of us without wheels.

Also, an event at Oriental Bay would have a higher profile, since a lot of people would see it going on, thus perhaps helping raise awareness and get people interested in further events at other beaches.