Because everyone prefers union workers, it creates a situation where the non-union worker has to get noticed somehow (nepotism or exceptional work) to convince someone to take a risk and hire them to earn enough work to gain union membership.
To users who are following along who aren't familiar, this is not how all unions works. Presumably the commenter is talking about "trade unions" which is one of many types of unions.
Everyone? What union are you talking about?
There's way more union members than there is work. So if the pay's the same for union or non-union (not much either way), why wouldn't you go with union labor?
When hiring union members, there's more paperwork to fill out and regulations that the production will have. For example, union members must be paid within a certain timeframe or else there will be late fees the production will need to pay. This is not the case with non-union members. Generally, it is a lot easier and less expensive to hire non-union workers if you can get away with it.