They don't think there's a market for Android because no one said anything. Android users didn't ask them to or enough Android users ask them. Or they think there isn't enough Android users out there.
They don't think there's a market for money (as said in above posts) for Android because they think everyone wants free aps (would
30 million people downloaded Angry Birds had it been a paid ap? not saying they're not making money off the ads, but several play in airplane mode or root to get rid of ads).
They think Android isn't long lasting, just a fad. I don't agree with that, but some people don't think Android will last, especially with the iphone on Verizon.
iPhone was their first app and with each new apple item they have to keep up with updating and changing the apps. An iphone app may have to be changed to work on an ipad and then for the ipad2, same with the iphone versions. So they don't have time to make Android apps.
And as said above, the time, knowledge and skills required to make it into an Android app.
Just some thoughts as to why...
Now some of these can be reversed to get Android Apps.
Letting developers know we want them on Android.
Paying for quality Apps.
Not saying you'll switch to Apple if....
Supporting Android devices.