Also probably that enterprises started migrating to Github. At the beginning it was probably early adopters platform with the languages that early adopters use.
Early adopters also use Java.
This is of course true in the trivial sense, but he means languages disproportionately used by early adopters (Ruby, Python, Go, etc.).
Yup, a large number of apps + a significant number of custom ROMs. It seems like GitHub became a standard for XDA-developers.