Speculative science fiction is not so much a window into the future, but rather a reflection of the present in which it was written. By drawing on current trends, authors can extrapolate a future. The cyberpunk aesthetic arose around the same time as the personal computer, early Internet, 80's corporate greed, continuing urban sprawl, the Japanese economic bubble leading to property purchases all over the world, etc. -- inspiration for many themes that appear in cyberpunk works. Many of these things have cased to be novel (in the same way) or exist at all. For instance, computers and connectivity is ubiquitous now, massive conglomerate corporations haven't totally taken over for governments (depending on who you ask, at least), China has somewhat replaced Japan, and the world has moved on. Unless they were just writing genre fiction, authors like Gibson would not keep cyberpunk going because the world today is very different from the one that spawned it.

Another thought is to compare it to the other famous "-punk" style, steampunk. Steampunk benefits from the fact that all the technological foundation (steam, mechanical, analog) it is based on exists. I'd argue that the genre can thrive because a well-defined foundation exists from which to create. With cyberpunk, some of the foundations exist (computer hacking and espionage), but many are still left as future speculation (synthetic organs, cybernetic implants). It becomes harder to have a consistent foundation when there are still moving targets. I guess we can still have trench coats and mirror shades ;)

I really enjoy the cyberpunk aesthetic, and I wish it had more of a footprint in current music/film/TV. It may be though that true cyberpunk is really a product of the 80's. The spirit of what drove it, technology-driven dystopia, continues to this day.