We DID go to conferences or expos for things that were buzzy and newsworthy, which meant the exciting new world of microcomputers: Comdex, MacWorld, etc. But the emphasis was on exposition, not conferencing with others, because we got to see exciting, new things with our own eyes, when that was the only way to see them (couldn't "see" things online). We wanted to see them (and they wanted to show them) to inform our decisions. The makers of C tools filled plenty of booths at the "computer expos", though.
Twenty years ago, we had started having big language conferences. JavaOne was like a religious revival or political rally, but C was the old devil being vanquished, not the Hope of the Future. And Visual-J++ and C# conferences were counter-political rallies, where C was going away as both sides agreed, but to be replaced by what? The makers of C tools still filled plenty of booths at the hardware expos, but there wasn't much demand for a C conference where we could hear, in person, about how C wasn't actually dead yet.
And by 15 years ago, all the hotness that inspires a crowd to gather for a conference was based on changes relevant to the WWW. If C had reinvented itself with shiny, new async networking, web service & e-commerce building blocks, concurrency, and whatever in its standard libraries or some such approach, there definitely WOULD have been C conferences. That would have been big news (and probably a bad choice, but that's a different question.) Without something like that, there is still plenty of demand for C, but little demand for a C conference.
Unfortunately that doesn't explain why there are not only aren't C conferences today, but there weren't C conferences 15, 20, 30 years ago.