[wilecoyote]

I believe part of the reason is that by suing Samsung, with direct (and substantial) revenue and profits from these devices it is more straightforward for Apple to demonstrate both infringement and damages. The case probably also becomes more straightforward for a jury. In addition, (though this may not be as much of factor here as Apple is simultaneously suing most of the Android device manufacturers) IP lawsuits often begin by targeting weaker parties with a greater chance of victory to establish early precedents before taking on larger or more difficult cases.