This is because video ads use an obnoxious ad API called VPAID  [1] that is mostly used to track viewability of video ads (which in turn conditions the billing event for advertisers). This is most effectively done with Flash and so 90% of video ads use Flash if given the opportunity.<p>[1] <a href="http:&#x2F;&#x2F;www.iab.net&#x2F;guidelines&#x2F;508676&#x2F;digitalvideo&#x2F;vsuite&#x2F;vpaid" rel="nofollow">http:&#x2F;&#x2F;www.iab.net&#x2F;guidelines&#x2F;508676&#x2F;digitalvideo&#x2F;vsuite&#x2F;vpa...</a>