An envelope is a container and has boundaries. Pushing the envelope is similar to "take it further", and is referring to extending the boundaries. Opening up possibilities.

It's a strange expression, isn't it? What it means is to extend the limits of some activity, within which people have generally had to operate. I think it may have come from aeronautical engineering and refer to some hypersurface in a multidimensional space modelling parameters such as flight speed, aircraft angle, air pressure and so forth, where that hypersurface defines the envelope in which it is safe to fly. (Or perhaps it has to do with some chap in the post office ... )