All those things you write in the 1st paragraph, I wonder if it's 9/11 or the continued spread of Internet access. Previously if you thought the earth was flat, your friends in school or work would laugh at you. Nowadays you can google anything and find a community totally into that idea, and they are indeed communities because they'll quickly indoctrinate that the rest of society/big government/the secret elite cabal are prosecuting them for their beliefs, so they'd have to support each other..

Oh yes, definitely the internet plays a huge roll in the world we have today, probably a bigger roll than 9/11. Humanity has seen and adapted to large scale tragedies before, but this is the first time we've had the internet. But of all the variables, 9/11 is a big one in the US. It also showed the media and advertisers how much more money there was to be made from these events. Ever since 9/11 the news media has been hard at work developing new ways to milk sensationalism junkies the same way that mobile companies milk gaming addiction whales.

Worth noting: most of us belong to several such communities, but we tend not to realize or consider it, as flat earthers do not realize what they do.