In QAF we already have so many visuals of all the pairings, the images from the show really ARE the way the vast majority of people see the characters.
In HP, the fandom has always seemed to me to be more book based than movie based, so I think fans are far more likely to have different mental images of the characters and want to portray them though their own art. And of course, anything involving the majority of pairings has to be completely imagined, whereas I can turn on QAF and see B/J fucking. :P