The entire Ubuntu Software Center thing started when Ubuntu MATE decided to allow the users to choose between App Grid and the USC by default, on their system.
Next, some developers announced that they would fork the GNOME Software Center, to use it as a USC replacement, but most likely this will not happen.
If you ask me, Canonical has ditched USC and will replace it with Software Center from Ubuntu Touch when the system will reach full desktop-mobile convergence.
Right now, Canonical does not accept DEB packages for new systems anymore, meaning that new software will not be added to the default repos from now on, at least temporary.
We don’t know what caused this or if it is an issue or something done on purpose, but we will keep you posted, as soon as new things on this get revealed, you’ll know.