Google is having trouble indexing these pages, which is why they don't rank well
Google has explained why, seemingly, direct iTunes links to various iOS apps were hard to find in Google searches. This led plenty of people to speculate that Google may be downranking iTunes links. The developers themselves wondered why the links were so far down in the results.Google says there are problems indexing these pages, though it makes it clear that the problem is not on its side.
The head of the web spam team and the de facto public face of Google Search Matt Cutts tweeted that Google is getting 403 errors. 403 (forbidden) is the HTTP error thrown by the server when you're trying to access something you're not supposed to.
"The iTunes web servers were sending lots of 403s when fetching from certain IPs," Cutts explained. "In other words, it wasn't anything on our side," he added.
If the Googlebot can't access a page it can't index, it is making hard if not impossible to determine its rank. It makes sense for a page that's not accessible to the Google crawler to be downgraded.