Because it results in a parasite effect. The union uses its power to negotiate a pay rise for its members. The business then gives the pay rise to all the emp

Because it results in a parasite effect. The union uses its power to negotiate a pay rise for its members. The business then gives the pay rise to all the employees. Each employee thinks “Well I would have got the benefit if I was a union member or not, so I’ll stop paying my union fees”