Great question. Here are the answers:
We all don't want the same things. For instance, some want a pension, whereas I would rather get all my compensation up front so I can plan my own retirement.
Unions seem to morph, over time, into a bureaucracy with its own agenda just like any other grouping of human beings.
Unions tend to try to coerce members who don't agree with the leadership.
Unions tend to slow the employer's ability to quickly adapt to changes in the marketplace, endangering the long term survival of the company.
Pilot unions tend to protect bad pilots who REALLY need to do something else for a living.
Unions tend to forget the company's prosperity is essential to the continued employment of it's members. NJASAP post Luthi does not seem to be headed that way, so far.
Unions sometimes strike, which drives me nuts. If I don't like my job, I will leave, not dreaming of pressuring others to strike.
When unions strike my company, voluntarily leaving their jobs, and I don't voluntarily leave MY job to join the mob, I get yelled at and called a "scab". Yet I don't mistreat the strikers at all because they don't agree with me.
In general, I believe humans behave better individually than they do collectively.
Mark Luthi. What a GREAT argument against unions.

I believe my way is not perfect, but it is better than the union way. Cheers! And thanks again for the good question.