Many developers, including myself, have wondered why Google doesn't put more emphasis on helping developers promote and sell their applications. At the launch of the Android Market, Google didn't launch with carrier-based-billing (they turned down this option, preferring to use their own Google Checkout at a later date). This decision forced many developers to give away their applications.
We all know Google would love to see more applications bought, driving in even more applications; but when it comes to backing this up with money and resources, Google seemingly falls short.
One problem I see is that there is no profit motive for Google to do so. They collect some small percentage with their Google checkout and then the carrier gets the lion share. So if sales of applications are poor, it doesn't affect Google's bottom line, it affects the carrier's.
We do, however, see a large number of free applications being downloaded. These developers are increasingly turning to solutions such as AdMob because sales of paid applications are so poor. And the more advertising banners that go into Android mobile applications, the more Google stands to gain, at the expense of the carriers. (The carriers get to pay for the network costs of the free downloads and embedded advertisements).
The next step is that the carriers will demand a percentage of the ad revenue and then everyone is now playing in Google's space. The problem we have now is that charging for applications is at odds with the expansion of mobile advertising. So why should Google put money and resources on something that is not in their long-term interest?