It could be a matter of resources. They have an allotted amount of dollars and manpower to contingency lawsuits, as has been learned early in the game.
Currently they are fairly busy with the remaining 5 suits out of the eight filed so far. We don't know, but the settlement phase, which we believe they are involved in with 4 of the remaining infringers, may be much more time consuming than the legal motions as they jockey for position.
My thinking is that when we see added settlements including very specific agreements and documents, then we can expect a good number of new suits to be filed, hopefully 10 or more.
Let's consider that since we have three actual settlements resolved, they could be working on the early stages with many more companies who may want to settle without incurring legal expenses.