Your post is like 2 years old at this point, but I figured I would come in here and post a solution that worked for me. I found this thread while searching.
It turns out that my monitors were set up as an extended desktop, so I could have more screen real estate space. if the physical premiere window is over the edge and even just 1 pixel on the other screen it disables the ability to output to the monitor. Simply enough I just moved the window over a little bit and the option came back in the drop down box.
(actually found this solution in another creative cow post.)