ROI - Return on Investment.
Meaning, how long before they start making profit. The app may sell for a buck or two, but it costs software companies thousands of dollars to make. They still come out with updates for the iPhone because there's a team dedicated to refining the app. That same team probably won't be the one making another app for the Android.
The Apple app store has a very fast ROI because there are soo many users purchasing apps. The Android market is slowly gaining popularity but still has some ways to go. This is why Gameloft announced earlier that they would stop all Android app productions (however they realized their fatal mistake and now decided to invest in Android).