Ubuntu MATE Is Dropping The Ubuntu Software Center
This Ubuntu "Software Store" has already been removed from the daily images of this MATE desktop spin of Ubuntu, ahead of the 15.10 Alpha 2. Wimpress mentioned they have "something else lined up by way of replacement" but that it's not the Debian Synaptic UI.
The original Ubuntu Software Center, back when it was called the Ubuntu Software Store prior to being renamed, back in 2009.
Martin Wimpress shared these details via this Google+ post. This decision only affects the Ubuntu MATE version and not other members of the Ubuntu family. The Ubuntu Software Center has been criticized recently for being bloated, not well maintained, and in need of a full rewrite.