Competition.
Sure iOS for now and the foreseeable future will bring in the most app revenue, but what matters is not the total but the per-developer revenue.
Currently iOS developers make the most money, but as the market becomes saturated it will be harder and harder to stand out among the crowd and claim your share of the finite dollars going towards apps.
Since Android has less app competition, eventually its non-saturated market will look better and better to developers. Especially if iOS grows oversees and developers from developing countries with a low cost of living are willing to undercut first-world developers.
Eventually in a saturated app market, developers won't be able to be so picky if they want to survive. They will need their apps on all platforms to pull in the same revenues they get today from just iOS.
We are living in an app bubble or an app gold rush if you will. In ten or even five years the market will stabilize to more reasonable levels and developers will learn to deal with the realities of the market of that time, just like real estate today or tech after the dot-com crash.
Basically now they have the luxury to avoid Android. That is temporary, like all market bubbles.
I think you are looking at the App Store as it was... two years ago.
It's grown to be the opposite of what you are describing. New apps and small-time developers get the equal amount of exposure as big league ones. Obviously not through the Top Apps lists, but through the "search" function in the App Store. For instance, if I were to search for "racing" right now on the App Store, it wouldn't bring me the best grossing racing games, or the best rated racing games as the top results. It would bring me a list of anything related to "racing" and down the list somewhere would be Real Racing and Need For Speed, which are arguably the top grossing games now.
My point is... all apps that are released at the same time receive the same amount of exposure. In fact, if you are a game developer, you can join a community like TouchArcade and declare your app's release date for everyone in that community to try out. It's free exposure for now.
Will this change in a few years? Perhaps. But if Apple keeps the search function the way it is right now, I don't doubt that my next great app can come after 10,000 other apps like it and still stand out.
If competition was so stiff with iOS, you'd think Indie developers would have stopped jumping in a while ago. But no, Apple still attracts more developers day after day. And that's something.
In a few years, would it be different? Yeah. But I don't think anyone can say for sure that by then the App Store hasn't grown into something that no one could ever match in terms of scale.
To me, looking at the App Store now is like looking at Windows when it was in 3.1. Eventually, it'll be 90% of the market, or more. And I know for sure that none of us can really say that Mac OSX or Linux will overtake Windows in the next few years, or decade.
Some things just grow really large a few years. I'm not saying that there is no chance of Android catching up, but with the current momentum, App Store would have left Android Marketplace far behind. And it's not because App Store is growing fast. It's just because Android Market is growing slowly.