There are exceptions especially in the public sector, but in the private sector more highly educated and skilled employees tend to be less attracted to "collective bargaining." They tend to perceive unionization as less professional and more generally prefer to rely upon their own "individual" talents, skills and motivation to satisfy their extrinsic and intrinsic needs.
As you move down the talent, education and skills continuum where such workers are -- on an individual basis -- less prepared and more abundant they may be more attracted to the concept of "collective bargaining" where they can possibly leverage greater rewards and consideration by threatening to withhold their services in mass.
Does this make sense?