They will when they're threatened by a gang of police officers.
Malpractice insurance is an important part of police reform but alone I think it will be insufficient. If you combine some sort of mechanism that sees lawsuits against police departments being paid out from the police pension plan instead of taxpayers coffers I imagine you would see police reform really quick.
It's all about aligning incentives and only when one shit-head police officer's corrupt behaviour is detrimental to the group will you see reform. Divide and conquer.