Gnome Software has copied a lot of features from Ubuntu Software Centre, but it still has a long way to go before it's a full replacement for it. When I tested it a couple of months ago it s till couldn't handle more than a small subset of the available packages.
To handle the majority of packages you had to install another GUI package manager such as Software Centre or Synaptic, or apt-get the package from the command line (assuming you know the name). And if you have to do that anyway, then why bother with Gnome Software?
People wonder why Ubuntu went off in their own direction with Unity. The reason was simple. Gnome was going off on a decade long wander into the realms of fiddling and experimentation, and Ubuntu saw what a train wreck that was going to be and hopped off at the next station. Whether or not you happen to like where Gnome 3 is going, it's pretty hard to deny that the way that the Gnome developers went out on that trip was appallingly bad.
Of course Gnome is controlled by Red Hat employees, and Red Hat has only a marginal interest in the desktop. For Canonical their Ubuntu desktop is a core strength and they weren't willing to risk that, hence their need for a "plan B".