If you search the forums you will find a lot of discussion on this, most of which is likely just hearsay, rumors, speculation and even a few outright fabrications. The "official" reason from Hobbico I believe was that Sig was having difficulty delivering products to them so they unfortunately had to part ways. As long as Sig stays around I wouldn't be surprised to see them back in inventory at Tower Hobbies sometime in the future. Even if they don't come back, there are other places to get Sig products including direct from Sig.

Dropping vendors or distributors is just a reality of doing business. It happens in every industry where products are de and sold. Unfortunately it isn't always popular with customers depending on the particular product line.