They didn't get that much attention in 2016 either, but to answer your question, the reason they got SOME attention (not a lot like you say) was because people were actually looking for alternatives. That wasn't the case in 2012 obviously.
Well yes, I know what the reasons are. But like I said, I think much of it is self-reinforcing. When people don't know who the third party candidates are, they don't think about voting for them. But when they actually get enough coverage to be included in polls, then people are willing to entertain the possibility of voting for them. They got enough coverage in 2016 to be included in virtually every poll from ~May onwards, so the media was talking about them, at least a little.