I think the main reason against making a WebGL 1 extension for something that is supported in WebGL 2 is because WebGL 2 implementations are already in the works, so this will happen eventually anyway.
The main reasons for including stuff into WebGL1 that's included in WebGL2 is:
Because it's obviously useful
It eases forwards compatibility issues for going from WebGL1 to WebGL2
It eases backwards compatibility issues for supporting WebGL1
Since WebGL2 will not be available for a long time in many places, it provides at least a path for UAs to bring more WebGL2 features to their WebGL1 implementations.