Several trends are helping push casual games out of the margins and into the forefront. Advertising has emerged as a key revenue opportunity for games, and hard-core games reach only one of many demographics that advertisers covet. Also, casual games tend to be small and have minimal processing needs, making them ideal for mobile devices, particularly cell phones.
While “casual games” don't usually have quite the quality or depth of more grandiose projects, many people aren't looking for that. They are looking for something to relax and be casual with, which is probably mind-numbing games like Solitaire are so popular. EA is one of the biggest providers of these “casual games” currently, and others are looking to start.