It's easy to assume all "pro-gun" voters would've voted against I-594 but that's unlikely to be the case. If you buy maybe 2 or 3 guns in your lifetime, and know that anyone you'd leave them to upon your passing, or to whom you'd sell them prior to that event can pass a BG check, why would you care about a UBC constraint?
I-594 passed with a 60% majority voting yes, you'd have to hope for some amazing odds from the "pro-gun" vote to turn that around. And, if something, or someone, had come along to fire up the many voters who stayed home, well, it doubtlessly would've excited as many "anti-gun" voters to come out too.
I-594 passed overwhelmingly because no one presented a credible, easily digestible case against it. The opposition was pathetic, albeit not for lack of effort. UBCs in WA and OR are the new normal, may as well get used to it.
For the record my family universally voted NO on 594.