'Not Invented Here' syndrome. A bad reason.
A fresh start. A project designed to their needs
That's shortsighted. Maybe the short term goals had different accents, but in the long run, Twisted and Tornado have the exact same goals.
Major kudos from a large community of developers interested in "real-time" web services and applications
For rewriting something that already exists and not doing an significantly better job and arguably a worse job? Applying those resources to Twisted would also have yielded this result.
Tons of free publicity to the project and FriendFeed/Facebook's open source initiatives.
Also belongs under A. Large contributions to OSS would yield that same effect.
A fresh start. A project designed to their needs Ability to lead the project in the direction they like
You are assuming, without any reason, that they wouldn't have had that chance with Twisted. If that wouldn't work out, they could easily have forked Twisted, effectively taking over the project.