Because Gorgol should be returning to Sauron after accomplishing a mission and the fastest way to reach Sirion Valley would be North-West. If he were on a spy mission to Doriath (e.g. to capture some Doriathian border gards for interrogation) he would not draw attention to himself by involving himself in a massacre in Dorthonion.Why would he go North West?
If there are any tracks to the South this would need an own backstory and an elaborate explanation. The orc road from the pass down is being build exactly to avoid this area and to bypass it from the West as there is nothing there but poison and spiders. The only reason for Gorgol to go through there would be if he were to defect to Thingol, but that is not reasonable to assume.I think if he follows tracks South then he goes South. If Sauron is trying to claim the land and sending out troops to build outposts then they that's where they go. Doesn't mean he doesn't find orcs skeletons plastering spider holes etc.