Thats an easy question to answer. As a union employee i completely understand how democrats are supposedly my best friend when it comes to my labor status as a union employee. The problem is that in practice that is anything, but true. First off lets examine union membership as a whole. Union membership has been dwindling for decades(peaked in the 1970s). That means that under democrat administrations and Republican administrations unions have made up fewer and fewer people in the workforce year after year. That statistic alone means democrats aren't our friends, but lets look over Obama's term in office to see if he was a friend of unions during his term. During his first two years in office there was a bill proposed that would have made it easier to unionize Fedex. That bill could have passed, but the democrats did little to support it. Since then I have seen little or no effort to further the union cause. Why should I support someone who never backs up their words with actions?
Personally though, I also have many other reasons to vote Republican. There are certain principles that I will not sacrifice like the right to free speech, the right to bear arms, the right to to be free of unreasonable search and seizure, etc. The democrats are on the opposite end of the spectrum when it comes to supporting traditional American ideals, and therefore I cannot vote against my great country just because I am a member of a union. I know the Republicans aren't perfect either, but atleast they have a record of standing up for these American ideals much more so than the democrats.
Finally I have to prioritize who I am. First and foremost I am a American, then a conservative, then a union member. I can always find another job, but finding another country like this one is impossible. I'll take my chances voting Republican because lets be honest here, they generally support everything that made this country great while the democrats support and represent everything trying to tear it down.