GitHub have changed their recommendation several times (example).
It appears that they currently recommend HTTPS because it is the easiest to set up on the widest range of networks and platforms, and by users who are new to all this.
There is no inherent flaw in SSH (if there was they would disable it) -- in the links below, you will see that they still provide details about SSH connections too:
HTTPS is less likely to be blocked by a firewall.
https://help.github.com/articles/which-remote-url-should-i-use/
The https:// clone URLs are available on all repositories, public and private. These URLs work everywhere--even if you are behind a firewall or proxy.
An HTTPS connection allows credential.helper
to cache your password.
https://help.github.com/articles/set-up-git
Good to know: The credential helper only works when you clone an HTTPS repo URL. If you use the SSH repo URL instead, SSH keys are used for authentication. While we do not recommend it, if you wish to use this method, check out this guide for help generating and using an SSH key.