Remember, there are also a lot of kids using iPod touches. They don't require a data plan.
This is a big part of it. I have (and love) an iPod Touch. I also really love my Droid. That said, here are some of my thoughts on the question.
In short, from a promotion and marketing perspective, you also have the marketing machine that is Apple behind it. Other points that occur to me, just from a marketing perspective:
-Direct integration into iTunes, which is a major media player app, even if you only count Mac users, which provides a lot of exposure.
-Greater promotion through other media outlets, such as Macworld, Maclife, PC World, etc.
-TV ads (sure, you see Droid ads, but they are focused on the phone, not the Marketplace).
-Greater exposure to foreign markets
-[FLAME SUIT ON] A MUCH more seamless App Store experience than what you have with Android. Applications like App Brain go a long way to improve this, but it's a separate app, a separate website, more button presses and steps to achieve the same end result. If you make a pure comparison between searching, purchasing, downloading and installing apps between the OFFICIAL Marketplace and iTunes App Store, it's not even close IMHO. Imagine explaining and demonstrating the two processes for a non-technical parent or grand parent, and think about which one they are more likely to prefer. Remember, geeks are great, but the non-geeks count for a lot of sales as well.
This might not matter to folks like us, who are into tweaking and participate in forums like this, but if you look at "average" users of both devices, and try to consider the "lowest common denominator" these are legitimate issues, both with exposure, marketing and ease of use, that will affect the number of apps sold.
That said, I am by no means a quantity over quality kind of guy. I really don't care what the total count is, as long as I can find good apps that do what I want them to do-and I am happy to pay for them if they are effective and good enough. You quickly get to a point where more apps just means more garbage to sift through to find what you're actually looking for. 100,000 apps in the App Store doesn't impress me, nor does 30,000 apps in the Marketplace, if that means it takes me extra time to sift through the spam to get to what I need. "More" is certainly not always "better".
