1. Twitter has nothing to do with early adopters anymore...they were on freaking Oprah.
2. They've been around for 3.5 years, which is only a couple years less than Facebook, so it's not like they're this little upstart that's trying to unseat the ancient Facebook.
3. Facebook has way more retention and engagement / user.
4. Facebook does what Twitter does (status updates), and does it better, IMO.
5. My parents and aunts and uncles and grandparents all use Facebook. They find value in it, and it enriches their life. Twitter is useless to them.
6. Twitter now is mostly marketers and spammers, judging from the people trying to follow me.
7. Most people seem to talk on Twitter way more than they listen. Over time, this drives down engagement, which means that people have to talk louder and more often to get any attention.
8. Most importantly, Facebook has hundreds of millions in annual revenue, and is predicting cashflow positive in 2010. Twitter has yet to make a dollar (to my knowledge).
EDIT: Added a few more points