Threading is good for reading through one time and when there are many topics of interest. Email is something you often return to again and again and topics are organised using subject headers. Email discussion view was introduced by google a very long time ago before almost anyone else did it. It is a bit of a cosmetic trick where it just sorts emails that have the same subject. Threading does not jive with the way Email technology is structured.
Remember when Google tried to build a brand new huge piece of technology meant to revolutionise communication on the web? Nobody adopted it, was called Wave.