Keep in mind, the union memberships are decreasing because family wage manufacturing jobs are going to China and other places. This, middle class income is declining too.
You can thank those supporting global free trade.
That had something to do with it, but not the greater part of it.
That would be the corrupting of union hierarchies by corportist special interests for the purpose of weakening them as effective advocates for their memberships. Money, the offering of positions, and other means of generating influence were used and, because many of these folks were weak, successfully. So the unions were infected without. The use of the same corruptive methods were used in state houses to pass "Right To Work" laws [aka, the right to work for what I want to pay you laws] and other measures to weaken them further from the outside. Together these outward and inward corruptive methods, combined in lesser measure with what you mention, has made all but the public sector unions benign, ineffectual, or dismantled and no longer in existence. Even those public sector unions are shadows of their former selves.
The reason for the wanting of this, besides controlling their costs and raising their bottom line, is simple. These unions were the special interests acting as a counterweight on corporatist political influence/corruptive practices. Their combined purchasing of influence acted against those of corporate interests and in equal measure. This balance of equally wrong, but diametrically opposed, special interests negated the presence of each other. It leveled the playing field. It was, in great part, why we experienced a period of middle class expansion and better governing. Not because there was no corruption, but because there were two major sources of purchasing who each wanted opposite end results of it, thus tending to cancel each other out.
The killing off of the unions is what allows the corportist influencers to run the table as the biggest, and heading to sole, source of corruptive influence purchasing. This lets them purchase more for less, and it is why they have the level of influence they have in our Congress and state houses today.
As I have stated elsewhere, this has happened here before. It will have the same end results if it doesn't change.