Given the large number of blue collar and service industry workers, why are labor unions relatively weak in America compared to in the past and in other industrializes economies?

It's because the globalization allows Companies in united states to give the job opening to citizens of other nations.

In the past, companies in America have no choice besides using local labor force that're available near them. This makes labor unions have more power in the negotiaion process.

Now, if the local workers refuse to work for a certain salary, the companies could easily throw the job to other nations.