Putting someone on the post is like admitting defeat, mind you. If your zonal and/or man marking is good enough, you won't need them.
I think it's changed for a few reasons. Firstly, the mix of zonal and man marking needs the players in other areas. If a team leaves a player up the pitch, that makes it even more difficult to put someone on a post.
Teams who bring everyone back do so with the intention of breaking out in a block when they clear the ball. Players on the post can end up too deep and play someone onside if a second ball comes in.
And the percentage of corners that need someone in that area is so low, teams are willing to take the risk to have a player in a more effective area.
Just my take on it. If it was somehow a huge mistake, you wouldn't have as many teams who don't put people on posts as you do.