OK, first off, there are no ‘rules’ in publishing, except that you need to enclose an SASE with any print submission. Saying there’s currently no market for something doesn’t mean that’s a rule. ‘Rule’ implies that it’s dictated by the publishing industry. The ‘no humour by unknowns’ thing is the exact opposite: it’s dictated by the market.
Most things in publishing are. Their whole raison d’etre is to sell books, not to come up with byzantine rules to keep poor aspiring authors unpublished so that agents and publishers can point and laugh.
There’s no market for books of humour essays by unknowns because when was the last time you went into a bookshop and picked up a book of humour essays by someone you’d never heard of? Humour is so personal and subjective that people tend to rely on known quantities.
The idea that you can’t get published unless you’re famous is absolute bollix peddled by vanity presses. If any supposed publisher or agent ever tells you that, put your hand on your wallet and back away.
However. In non-fiction, it’s easier to get published if you’ve got platform. This isn’t the same as fame. It could be fame, sure: if you’re a movie star, your autobiography will be an easier sell than Joe Nobody’s. Or it could be qualifications: if you just built a Formula One racecar out of tin cans and won a huge race, your Build Your Own Car book has a better shot. Or it could be an audience, offline (you’re the expert who gets asked to speak at every single Star Trek convention on some specific aspect of the Star Trek universe) or online (your blog on cooking without dairy has four bazillion hits daily).
In fiction, platform makes basically no difference at all. If your book is good enough, and it’s in a genre that’s currently marketable (serial killer crime, for example, went through a huge boom in the 90s with Patricia Cornwell and Jonathan Kellerman and people like that, but it’s fallen off an awful lot since then, so a new serial-killer writer would have to be really, really good), then it’ll find a home.
The main reason why books don’t get published isn’t that the publishing industry hates unknowns. Agents and publishers love unknowns: it’s a lot easier to turn a first-timer into a huge bestseller than to do it for a mid-lister whose last four books sold maybe 20,000 copies each. The main reason books don’t get published is, overwhelmingly, because they’re not good enough.