Voting is open to all attendees of the World Scifi Convention and they usually get thousands of votes now. It's possible that you find the winners too "mainstream" but accusing them of being cliquey doesn't make sense.
WorldCon is not representative of all readership. Its website even notes: "The Hugo voters are good at finding and nominating good works, and do talk among themselves, so word spreads." [1] They seem to do an awful lot of talking among themselves, I'll give them that.
Besides, it doesn't take a whole lot of nominations to get something onto the ballot. An energetic and motivated clique can capture the whole thing.
And surely it's apparent that they reward a certain type of work, and disregard others. For e.g., Analog Magazine -- which specializes in hard science fiction -- didn't pick up a single short story or novella nomination.
Traditional print magazines like Analog are at a huge disadvantage in popularity contest awards like the Hugos because they are only available to subscribers.
Hugo winners usually come from magazines that make their stories available for free online.
