New York used to be the big SF hub. However, around the 70s you had some fundamental changes in Fandom. Remember that many of the magazines were published in New York and by New Yorkers. In the late 60's/early 70's however, Cons grew in popularity. At the same time there was a population shift moving across the country. New York fandom simultaneously was growing in worse odor for a variety of reasons that I don't feel comfortable discussing here (but it wasn't entirely unjustified). By the time the dust settled, many of the SF authors lived in Cali, most of the magazines had shut down, there were far fewer fans in New York while many had migrated to California from all over the country. There weren't that many Fans left in large enough concentration outside of Cali to organize cons, let alone the ability, inclination. First Fandom was mostly dead or retired somewhere warm, the Baby Boomers were in their 40s and 50s and too busy trying to survive. Meanwhile Silicon Valley meant there was a large influx of wealth in California and many of the folks here had grown up on the stories that inspired them and wanted to meet their heroes.
In recent years as crypto fans have begun to come out into the sun, more and more conventions are being found everywhere with each year, and all are growing larger. So while there are still many in California as there have been for many years, the disproportion is beginning to delocalize.
To summarize: there was a SF diaspora that mostly ended up in the rich fertile fan-soil of California. That's beginning to change.