"in front of us"
We aren't given a why for this, but I have a hypothesis. -maj indicates possession, while maH tlhop is a more general genitive meaning which might indicate possession. Klingons may not consider a location relative to your position something you possess. In this case saying maH tlhop would mean it's the tlhop associated with maH, not the tlhop possessed by maH.
We are told that Klingons of the Sakrej region of Kronos have their own dialect of Klingon, and in that dialect they would, in fact, say something like tlhopmaj. Using a word like this would mark you as coming from the region of Sakrej or mimicking their dialect. Using a non-standard dialect outside of the region that dialect predominates is generally considered inferior. You'll be understood, but looked down upon.