In early 2011, Family Guy creator Seth MacFarlane announced he would be reviving The Flintstones, but for a Fox audience, as he was a fan of the original series from the 60's, and that the first episode would air in 2013. Later, in 2011, Fox aired a commercial hyping up the show's premiere in 2013, which featured MacFarlane talking about what he plans on doing in the reboot: https://www.youtube.com/watch?feature=player_detailpage&v=BJs5WVX7JC0
After that however, little was known about the project for a while, including if it was still being developed or not. Production of MacFarlane's 2012 film, Ted also made people believe it was cancelled.
When asked about the progress of The Flintstones at a convention to raise awareness about Ted, MacFarlane confirmed that whatever he had complete was shelved because Fox didn't think that a version of The Flintstones for an adult audience would be appealing to audiences. The plan, and whatever was complete has stayed that way since.