Because it's hard to get people to enter that information. In contrast, Harvard students in 2004 were highly motivated and that started everything off.
From 2005 to 2008, the killer feature of Facebook was entering your real information when signing up and then Facebook showing all the real people in life that were connected to you. E.g... old high school friends finding each other again. Likewise, signing up for Facebook means you also wanted to "be found" by others. It was the first mainstream service to successfully accomplish this. Myspace didn't do it. Friendster tried but their webpage performance was too slow.
>The other thing that drove adoption was the relative simplicity of publishing content.
Most Facebook users do not "publish content". Most just follow their friends or read links passed on by friends (or the algorithm). [EDIT for clarity: I mean "publish content" the way that gp I replied to cited "blogs/personal websites"]
By publish content I mean post links, photos, comments, rants, etc. Do people no longer do that on Facebook?
Finding people you know on Facebook is only one aspect of their success and it's probably the most heavily network dependant one. The other thing that drove adoption was the relative simplicity of publishing content. Your Facebook profile was/is essentially a dumbed down webpage where you can post text, links and photos. You also get free hosting, free "email" in the form of messaging, and a bag of other features that you would normally have to set up yourself and pay for. They did a good enough job of providing all of the killer features of the internet for free to average, non-technical users.