Nothing. Most people are just not interested in large organized activities that take significant amounts of time. Even with Flex and cross server grouping, people still don't want to do it en masse. They would rather do tourist mode in LFR.
If I really think about it, it's typically the raid leads, the unfair loot systems, guild rules, and all the minutiae involved to do it is what puts many off of it. That along with difficulty. And please don't go into the people have no reason to get better with LFR. They had plenty of "reason" from vanilla through LK, and still no one raided.