With the changes made to kickoff rules this year, kickoff returns only occur maybe once a game. This effectively cuts the amount of ST coverage plays in half. This is probably the biggest reason for semi-dismantling last years dominant coverage squad.
A lesser reason is probably that they valued youth/potential over ST coverage players. We have lots of young potential at certain positions (WR/RB/DL) and they decided that is worth more than the punt coverage unit.
Anyway, these are the tough decisions that good teams face. It will just get worse in the next few years, eg "resign Crabtree to a $60M contract?" etc.
You can always second guess the FO, but in this case they made a well thought out, rational decision. (well thought out decisions were sometimes lacking in years previous)