I’m surprised every time this happens. After all, all the costs for a conference are known ahead of time - you know how much you pay for venue hire, catering etc. It’s not hard to run a budget and see where everything is up to. I’m doubly surprised at this as over-selling the workshop should have made it more profitable. But I don’t know what happened elsewhere in that budget.
There is something we can do as attendees :) If you are thinking of attending an event, register early. Stop leaving it until the last minute. It’s practically impossible to make a call on whether to cancel or downsize an event when 1/2 of your attendees register 2 days before. I made 2 losses last year because I assumed registrations would come in at the last minute, and then they didn’t. But usually they do. (and I still paid everyone - I wore the consequences of those losses)