because the GNOME2 code works great now, just the way some people like it. all it needed was some names changing to avoid collision with gnome3.
The GNOME3 classic mode is new, less well tested code, with less features and new bugs. it could be made to be a good replacement for GNOME2, but that might take a few developer years of work.
Ubuntu have put some nice work into GNOME3 classic, bringing back things like the system monitor. and there is the cinnamon project. so maybe in a few years GNOME3 will be able to do what want, but for now MATE just works.