The problem is pretty simple.
Most unions are not about workers, but are about politics. Nationally speaking, about half of the population aren't going to agree with the politics of most unions.
So, there is half of the support down the drain.
Many in the remaining half may not believe that politics belong in the workplace, even though they might agree with most of the union's positions. So vanishes another chunk of potential supporters.
Finally, there are those unions who are motivated by greed to strike at every opportunity, and they help create a hostile work environment, killing off more supporters.
So primitive, there's your answer.