Running two versions (releases) of WordPress puts undue burden on the developers. Now you have them spending time pushing things twice, and that will make releases take longer, and the eventual actual removal of code becomes more complex.
It's one of those ideas that is awesome on paper, but unless WP adopts a Drupal method of updating (that is there IS no real upgrade path from 7 to 8, just a migration page, and your themes and plugins won't work at all), then it won't happen.
We did rather paint ourselves into a corner by committing to backwards compatibility, but given the costs and the price, it's still looking like the right call.