I suspect in large part because Rackspace are much better at marketing, or were quicker off the mark than Amazon. They realise that an "en vogue" name like git

I suspect in large part because Rackspace are much better at marketing, or were quicker off the mark than Amazon. They realise that an "en vogue" name like github looks very good hosted with them, and are likely offering a big discount in exchange for the "Powered by..." banner at the bottom of every github page. We do the same for firms wanting to host with http://bigv.io/ http://bigv.io/ though I'd not imagine github would want a UK-based host.