why does a 3-1 surrender vote not pass?

If the other person has disconnected, then it'll only require 3-1. If it doesn't count them as a leaver yet it won't though, so you'll either have to wait for them to come back or wait for them to be discounted.

If you're simply talking about one person not voting even though they're their, well, they still count and it's not up to you to assume their failure to vote means they don't want to play. If they didn't want to play they wouldn't have queued up. It's most likely that they moved the vote window off the screen because they don't want to surrender and are annoyed by the damn box being in the way. Until they vote or are counted as absent, 3 is not a super majority.

actually, I remember one game where there was a 3-0 surrender vote on my team-->nothing happens. then we had a second 3-0 surrender vote (no afk's) and it actually went through.

I was one of those who just didn't vote.