I believe that when someone attends this kind of events and is not organizing them and/or paying for attendance with his own money, the smallest and kindest thing that can be done would be providing at least some kind of feedback (ideally some kind of report, of course). If they aren't willing to leave structured feedback, they could at least leave a line to explain "why" briefly.
Topic on Talk:Wikimedia Developer Summit/2016/Lessons Learned
Agree. :) What are you suggesting that they leave the "why" line?
Sorry.. typo... WHERE are you suggesting that they leave the "why" line?
It could be a different form, or a reply to your survey email or reminder, or a specific field on the main survey.
Of course this needs to be done in a way that doesn't encourage anyone to choose it as a shortcut.
If you had a way to find out why people didn't answer you, that'd be great. Maybe our surveys are just too long or something.
Would an idea such as a short "feedback sprint" (at the venue or later) be unfeasible/unreasonable? If we feel that hearing from more people would be valuable, maybe we should provide options different from "a long form to be filled online".
At wmcon I was involved in survey-related conversations. I learned that any survey should be filled in 10 minutes ideally and not have many similar questions. I'm guessing the events ones are already designed to respect these principles.