There are of course many reasons as to why this may be the case. Coaches may see two players not marking anyone as a waste of on field resources and may position them elsewhere. Some teams set up with 2 players in line with the posts but on the edge of the 6 yard line. This could be because those coaches believe that more players contesting the ball would equal less chance of an attacking player getting a shot off. This takes care of the problem of before it occurs. If the defensive team heads the ball away then there is no need for players on the posts.
Another reason I have seen thrown out is regarding offside. If you have one player on the post every opposition player would be onside following a second ball situation. Whereas if all defensive players were all on the 6-yard line at the very deepest then any attacking player drifting into the 6 yard box would be offside and the defensive team would have won a free kick.
Lastly and most intriguingly for me is that teams are now setting up for counter attacks from opposition corners. Rather than placing an emphasis on the defensive part of the corner they are already planning what to do once the ball is won. In this situation those two players who could have been on the posts may be positioned just outside the edge of the box or even higher up the pitch. This is so that there is an overload of players ready to counter when the ball gets cleared from the corner.