- They don't want to use it because they don't understand it.
- Facebook and texting takes care of the "what are you doing?" question just fine.
- Their parents use it.
- It's predominately used by marketers, brands, social media geeks (like me), techies, web developers, niche content creators, etc, i.e. not the young crowd.
- They aren't grown up enough to understand the value it can bring for business purposes BECAUSE they are mostly still in school or just starting their professional careers (do you see them flocking to LinkedIn?).
It's really that simple. When Gen-Y starts talking down to the next generation about "when I was your age", they will fully understand Twitter. The biggest reason is that they will be neck deep in their careers and see Twitter more as a business networking/marketing/sales tool than a social networking tool, i.e. see value in using it versus using it just for fun.
For a Gen-Y perspective, check this out.