Because cities were stupid enough to practically give the unions all the wanted when times were good. Now that it's time to pay the piper pay and benefits for these same employees sometimes reach 80%-90% of a municipality's budget.
Not many other places to cut.
There are several problems with public unions but IMHO they boil down to politics. The public union bosses provide the politician, local, state and national, two things the politician lusts after - money and manpower.
The money comes from the dues collected which first comes from the tax payer. This, again IMHO, causes no problems if the money moves between the political parties. But, as it has been reported many times it goes to and stays in one party - Democrat.
Manpower comes in two forms. Actual man-hours spent supporting a candidate. Again, I have no problem if the support moves between parties but it don't. The second form of manpower is block voting. While the ballot is supposed to be secret who out there wants to bet the farm on it?
With the power that public union officials wield I don't think there is a simple solution. Having said that I think the recent Supreme Court decision that allows corporations to contribute to political campaigns could be an excellent counter balance to the corrupting political power currently exercised by public unions.