It is my impression that Soarin' is genuinely popular with the general public.
An entirely anecdotal impression, of course. But a pretty strong one, if I judge the excitement and oohs and aahs correctly. People really, truly love this thing.
Not so in DCA however. Which shows that there is a large influence of particular audience, in-park location, presentation, hype that makes up an attraction's popularity. For one, it is also my distinct impression that lines create lines - 'there are always long lines, therefore it must be good'. And the reverse (poor overlooked Horizons!)