I think you're oversimplifying again. Many workers who vote "no" and reject union representation believe that unions create problems for workers including the problem of jobs being moved elsewhere.
Also, does "no impact" include workers paying extra ~$1000/year in union dues and the possibility of not having jobs? Many workers don't see the logic in paying more money to make their lives worse. E.g. Why pay my Bakers union rep to play hardball with Nabisco management for high wages of $26/hour so that I end up getting $0/hour? The theoretical "win" of $26/hr becomes a real-world "loss" of no job which pays $0/hour.
There are interesting game theory dynamics of "workers vs unions vs management" that your comment ignores. Each party has different leverage and different ultimatums. A bakers union is in a more precarious economic situation than a teachers union.
>A trade union has in fact control over important aspects such as wages.
Yes, I already stipulated that. Your comment ignores the other things that workers consider besides wages -- e.g. no jobs.
It seems you're not aware of the reasons for workers to deliberately reject unions.