Generally unionization is fought fairly hard by companies for the simple reason that it restricts what they can do and takes some control out of the hands of management. In the Bay area in the late 80's there was mumbling about unions after the great semiconductor flame out. A lot of people were laid off without warning and since it was widespread the ability to walk across the street and get rehired was limited. Since you can't really unionize when you aren't working somewhere, and when the people working somewhere are just glad they weren't the ones laid off, its hard to convince them they need to come on board.
As employer abuses, such as the 'no poaching agreement' came to light, or sudden changes in salary compensation etc. That provides incentive but it often isn't enough incentive to start the really vicious knock down drawn out fight that unionizing would entail.
Bottom line I think it would take a perfect storm of events, gross employer abuse, a large group of people who feel they have nothing to lose, and the prospect of a very bleak future unless something radical changes. We saw what happens when those forces align last Noveember, it doesn't happen all that often.