I wonder if one of the reasons people weren't inclined to attend is that it really was sort of an inappropriate celebration. Like, too fancy for the life change; wrong group of people invited, since they're not really THAT close to her; etc.
Or, "it's going to be at a venue that costs $XX per person, and we're all going to pitch in for the honoree's meal, and a present, and afterwards go drinking and dancing at Some Place.."
Most people I know have limited entertainment budgets, and personally, while I'd like to attend more things, I pick and choose the ones I'm willing to spend money on.
As for "but you have a husband.." I've been at the receiving end of "But you don't have kids" or some such implication that this means I can drop everything and work that extra overtime shift. As the OP pointed out, it's presumptuous to assume anything about another person's obligations. I like the idea of shutting them down with, "Yes, I do have a husband. Like I said, I had other commitments that evening."
So, the event itself wasn't really the issue. While we do tend to do more lunch time "good-byes" and those definitely get more turnout, it is not completely unusual for someone to arrange for after work drinks instead. Since I didn't go, I don't know if our ex-coworker's drinks/meal whatever was covered by the attendees or not.
I did not go because the honoree isn't someone I'm very close to, I wasn't overly impressed with the work she did (though it wasn't horrible), and because I had a commtment with my DD. Had either of the previous two reasons not existed, the third may have been overcome. I imagine that the others didn't rearrange thier schedules for her for similar reasons as mine. I do think, in this case, had she wanted a larger turnout, an event during work hours would have gotten it for her.