my guess, in a word, "marketing".
i haven't downloaded, and haven't played, but i'll still offer $0.02 fwiw because i think you're proceeding from a false premise: there doesn't have to be anything wrong with an app for it to do poorly. i don't know what marketing you might have done already, but i'd guess it wasn't enough.
marketing is vastly more important than quality. (oooh, heresy, let the flame wars begin) otoh, a "junk" app with good marketing can outperform a great app with no marketing.
that's not to say that quality doesn't help, because it'll help generate "buzz" if ppl like it - but again, that becomes marketing factor. ie, quality helps to the extent that it creates a positive marketing impression, and potentially build up brand loyalty for your next app, etc -- but quality alone does not sell. the corollary is: if no-one knows about your app, then it doesn't matter even if it's the best-app-ever - it won't sell, because no-one is buying. to "sell" you must 1) attract customers, and 2) convert them.
graham has already pointed out icon, video, screenshots. also you need a fuller description to hopefully get some seo discovery. and if anyone does ever find it, can they then try it to see if it's buzz-worthy? you've sort of built a $0.99 brick wall in front of it. maybe change from paid to ad-supported/no-ads iap, or separate free/paid versions. (check out your competition in the arcade "top free" category) it also seems a bit "heavy" at 21M for what it appears to contain. where's its website? its twitter/facebook? it's dev blog? have you posted about it anywhere? (outside of dev channels like here, which don't count) submitted for reviews?
et cetera - all those types of "PR" steps are pretty much mandatory for success these days. i myself am not a master of marketing either, btw, but i recognize its importance.