[iamtherockstar]

Android gained popularity because it was tied to Calendar and Gmail. These two services were pretty pervasive already, and so became the default in Android. When that happened, those two services became crucial to Android.

The fear that Contacts and Calendar will go away like Reader did (or, rather, will) is irrational. Reader barely had a mobile client version of it. With Google reigning in the branding on Android and requiring that it be tied even tighter to Google services, Gmail and Calendar became a dependency of Google's mobile OS.