The Galaxy was superseded in the future. Why would they outfit an old ship in an era of a replacement for that ship. They would simple stick their nose gun and extra engine on a sovereign. As Gene was wont to do, if it's old and the new stuff supersedes and contradicts it then the old stuff is no longer canon.
Because we've pleanty examples of starship frames being in service for decades, and it presumably makes "economic" sense to add a third nacelle and a cannon to a Galaxy class than build an entirely new Sovereign?