They made changes to looking for raid. So, it was less rewarding and players would try higher difficulties this didn't workout. They had a lack content outside of raiding because they had to train new employees. Legion on the other hand has a ton of content to do outside of raiding. Looking for raid isn't a lose, if you wouldn't have raided without it. Warlords lacked content and we had a million times more things to do in Mist compared to warlords.
You can run older raids for mounts, and vanity items. Those older raids are ran in looking for raid for people who are gearing up. We used to have a reason to rerun dungeon it was called valor points and valor point rewards but we got no replacement. We do need more reason to run older raids outside of mounts but people do run older raids through looking for raid or they pug for old achievements and mounts.