Ubuntu said in a blog post that the logo change came amid the ongoing evolution of their open-source software operating system.
"[...] there is nothing wrong with the old Ubuntu logo to my eyes (it’s certainly not as dated-looking as the original CoF from 2004) but I can’t argue that a bit of attentive upkeep is healthy. It helps keep things fresh and pacy," wrote Ubuntu's Joey Sneddon. "Ubuntu’s new logo was designed by the same person as the 2010 revamp, Marcus Hallam. He explains that although he and Canonical CEO Mark Shuttleworth explored some ‘left-field’ ideas explored they ultimately felt '…updating the Circle of Friends to a more contemporary look and feel' made the most sense."
Source: omgubuntu.co.uk