To be honest, I have only 3 Bagua Sessions and and one Hsing class, so I am in no way an expert. You might be better on Rum soaked fist, but here goes:
It is very very difficult to master any of the Chinese Internal arts, so getting someone who has a degree of mastery in one is rare. Getting a person who is a master in 2 is incredibly rare.
I was told by a Taijiquan teacher that they were shown some Bagua and Hsing because they had "gaps" in their training, so there might be something to that.
Another issue with Bagua is that it might be one of the most complicated systems you can learn. Even more technical than Taiji or BJJ IMO. I suppose a Bagua teacher might try and do some Hsing I once in a while to introduce something to students which is much more accessible and easier to pick up.
Another thing is that the Neijia arts is that it is much much harder to tell how well you are doing them. In Karate, if you have to do a high roundhouse, you can visibly see and feel your progress. This is much harder to detect in internal arts IME. It can be quite frustrating.
So IMO it is much easier to dupe people in Neijia arts as it so darn hard to tell if you are doing them properly or not. So if you told a noob they are doing Bagua when they are doing Taiji, how would you know the difference? In the same way, it might sound more impressive if you are teaching "Bagua and Hsing I", but would you really know the differnce? They could just be teaching you Bagua.
You said yourself why doesn't a Goju school teach Shotokan. Well, if you had to go a Karate school and you knew nothing about Karate, what sounds more impressive? A school that teaches just Goju or a school that teaches Goju AND Shotokan. The latter might still only be teaching you Goju but they call it Goju and Shotokan because it sounds more impressive.
"Let your food be your medicine, and your medicine be your food"