I was wondering if there might be some sort of sampling bias at work. It sounds like it still might be, although you mostly have those bases covered. Specifically, people who already use the service know where the feature is, so they know what they need to get rid of to find it. Perhaps more importantly, they know that the feature exists, so they'll be more persistant in finding it. Not having/finding a skip feature is probably a dealbreaker (it certainly was for me, until I came back and saw this), so it would just manifest as some new people not sticking it out for very long.

But maybe that's not it. My other theory is that the less adept a computer user is, the less they're able to ignore popups like that. I've noticed that many less experienced computer users have an almost pathological need to get rid of any sort of dialog box, message, popup window, etc. More advanced users tend to just ignore anything that isn't directly interfering.