Could be a lot of reasons, even ones that we might never be able to realize from our side.

We have a good scam busting community in this forum too. Going with the fact that 99% of the altcoins being posted on the internet are going to be scams and pure vaporware, the scam busters will likely detect them and call them out making their reputation drop down to a negative within a couple of days of creating the ANN.

Again creating a pseudo-community on social media where stuff is not that regulated is easier and therefore the scams work better unless they get reported. Even at times no action gets taken and they keep running the scam until one fine day the fools realize how much they lost.

So my assumption would be - being detected as scam/vaporware on bitcointalk is far more common than on social media.