3 reasons that it SEEMS as though this concept is more popular:
1. It isn't more popular. It only seems that way, just like it seems as though we have more natural disasters now than before - we don't. What we have is more connectivity, more access to information, and more ability to interact with people in distant places.
2. People who are not educated on a topic will often accept an argument from authority. If you sound like you know what you're talking about, and make statements with confidence, you can be 100% wrong and still convince a percentage of people to believe what you're saying is fact. Communicating online makes it harder to judge the authority of a person saying something, as you have no visual or tonal cues to determine if they are lying. Anyone can SOUND right on the internet.
3. On sites like this and others, the flat earth is probably the very best topic to use for trolling. Want to suck educated people into an online argument that is pointless? Start denying that the earth is a sphere. A second, nearly as successful topic is claiming that the moon landings were a hoax.