Even before the cell phone, I would guess that younger adults were out of the home more, and less likely to receive the pollster’s call. Now, its easier than ever.

Also, in general, people travel more today than in previous generations (work, pleasure, school, etc.), and mobile devices reduce additional barriers to travel.

Even if someone has a land line, they might not even use it for calls (internet), or they might just use it as a backup if they live in an area with spotty coverage.