Why Did Lindsay Clancy's Jury Lose The Middle Ground?

There used to be a middle ground in the Lindsay Clancy case. As of July 9, there isn't one anymore. 
Prosecutors dropped all three strangulation charges against Lindsay Clancy that day, calling them redundant. What that filing actually did was erase the space between the two remaining verdicts: life without parole, or not criminally responsible. No compromise. No lesser finding for the jury to land on if they're not fully convinced either way. 
Lindsay is charged with three counts of first-degree murder in the deaths of Cora, five, Dawson, three, and eight-month-old Callan, at their Duxbury, Massachusetts home in...