CDNs are only useful for things that can actually be cached, and which have been accessed recently. Static content is easy, and I'm sure Github does it, but dynamic content doesn't benefit as much for a CDN.
As another note, anyone doing a DDoS is going to target parts of the system that are more intensive. They'd just skip the cached things and go after the ones that have to be regenerated.
That makes sense, but can't a CDN also help identify the DDoS and block communications from certain servers. I remember seeing a captcha on Adria Richards website after she got those two devs kicked/fired. Maybe GitHub just needs better tools to identify and block the DDoSes quicker.
