I'll admit I'm not very knowledgeable about the logistics of unions, but I imagine there's some kind of up-front legal overhead. Like, maybe there are contracts between the union and the company, or within the union itself to determine how it's governed? I also get the impression there is some overhead in an ongoing basis in the form of extra meetings, more complicated negotiations, etc.

All of those things seem like relatively minor costs when spread out across a large employee base, but I could see it being prohibitive for a smaller company. Or maybe this is a solved problem and you can just find some boilerplate stuff online that takes care of the whole process.