I have no idea why, but it took Twitter.com almost 8 years to file a UDRP over a very popular misspelling of its domain name.
The domain name Twiiter.com was registered under privacy in March 11, 2007.
The domain since at least 2011 according to Screenshots went to a site that had the same colors and feel of Twitter but offered users “prizes” for clicking on certain links or providing information.
According to Alexa the domain has a worldwide ranking of 273,000 meaning that someone has been making a lot of money off of this domain for a very long time.