Because it is a negative reinforcement loop. Good teachers will self select and leave on their own because they can. Since Newark isn't going to be a desirable district to work in it is that much more important to actively remove bad teachers and provide incentives for the good ones to stay.

It's not that cut-and-dried. My own high school had both very good and some rather bad teachers. The union was a stabilizing force that good teachers used to ignore everything but helping their students while the bad teachers used it to bide time until they could hit the magic year for their pensions.

I don't think fixing the problem necessarily requires destroying the unions or just allowing school administrators to just fire people willy nilly but the problem will not be fixed unless there is a measure of accountability.