I suspect that we didn't get enough Ember responders, because I do think they have a reasonable sized community. 9k responses is likely not enough to draw a reasonable conclusion that it isn't popular. I wish we could have paired up npm downloads or some other stat besides some of this data...
Ember has a pretty large community—the framework has a few conferences every year that are filled up. Most Ember devs don't seem to participate in a lot of JS community things and are very quiet compared to say, the React community.
Trouble is with angular 1 is a lot of people are probably still installing via bower, angular 2 might have a lot of people installing via jspm, elm uses scoped packages which still don't have stats...
What kind of was a head scratcher for me was that Ember had such a high rate of "heard of it, not interested" from the responders. Especially when I looked a couple columns over at the Angular 1 data where a lot of people tried it but would never try it again.
I don't know how a sample of 9k maps to the developer community at large but I found it puzzling that so many people were dismissive of Ember. What makes Ember unappealing enough to not warrant being tried out in a side project? Personally, I saw it and thought "boy, a framework that wants to be stable for several years - sign me up."
I think it's because Ember forces you to do everything The Ember Way. Sure they're technically writing JavaScript, but it only works in the context of Ember. Ember makes it extremely painful to use standard JavaScript libraries so it's not surprising that Ember developers don't respond along with the rest of the JavaScript community.
