When I want to choose a VSTi in the VST Rack (F11) I get new cool Search pop-up menu, but if I want to add an Instrument track I see the old-fashioned menu? The same happens when I want to add a FX channel… 
I noticed this too, it seems not everything is streamlined and consistent throughout. I can’t see this being intentional, as I so no reason why the new drop down menu couldn’t be present elsewhere throughout Cubase. Perhaps this will be remedied in a future update, as I too would like to see the new menus elsewhere. They are much nicer looking, take up less space and are far more conveniently laid out with the search capabilities etc.
Wait until 7.5 is out 
Better you choose them via channel editor or mixer. There ist the search menu too.