Personally I think its the compatibility breaking changes. Basic behavior of the languages are also changed, not specific to the interfaces. So a lot of libraries only work with 2.7, some only work with 3.x.
Since its not a compiled language, your software may or may not work depending on the interpreter versions your user is running.
No reason to use 2.7 anymore or even sub 3.7. More software are taking advantage of new features and become incompatible with older versions.