Public-sector unions now account for more than half of union members, while the private sector accounts for less than half. Why has this change occurred?
There are two main reasons for this.
Unions have weakened in the private sector because of competition and globalization. This has caused many American manufacturing jobs (the kinds that were once unionized) to move out of the country. Those jobs that have not been moved have tended to become less unionized out of a need to compete with foreign companies more than was necessary in the heyday of private sector unions.
Conversely, the public sector has not faced such pressures. There are no competitors for the government jobs that are unionized. Furthermore, unions have some political influence and are therefore able to push politicians to accept unionization in ways that private sector unions cannot do.
For these reasons, public sector unions have done relatively well while private sector unions have declined badly.