[baru]

This was already answered on quora by PJ Hyett, GitHub Co-Founder:

"IO performance is essential to our business (handling an enormous amount of disk operations) and EBS wasn't nearly as fast as bare metal when we ran benchmarks prior to moving to Rackspace."

(from http://www.quora.com/GitHub/Why-did-GitHub-use-Rackspace-ove...)

While price most probably played an important role too, as indicated by EngineYard, its former host: http://www.engineyard.com/blog/2009/engine-yard-and-github-t...