Bandai have been cutting back lately in terms of providing subtitle only releases (Lucky Star OVA and Kannagi), and rightly so too in these times; demand for dubs is low anyway.

In this case, they are cutting down on boxsets.
If there are only going to be 2 boxsets instead of 3 this time around, cutting the pictures dramas makes sense as they have to fit the content that would have made boxset 3 into the others. Each season has the same amount of episodes after all.

This is a guess though, a pre-order could appear for a part 3 at any time; though it's unlikely at this point now.