Two reasons:
- I don't need another Facebook
- I get so much connection SPAM from LinkedIn that I consider LinkedIn one of the biggest cesspools on the internet. (Mostly recruiters who I've never worked with trying to connect, but sometimes people trying to connect as a way to promote their business.)
If they (LinkedIn) better policed connection SPAM, I'd use it. Maybe I'm in the minority; but constantly getting connection requests from people who I don't know is a major turnoff to me.
That being said, IMO, the observation that LinkedIn has a good tone to its conversations makes it an attractive alternative to platforms like Facebook where the tone turns toxic.