VNs are NOT limited to a highschool setting. It's just that those settings are very common, especially in moege. And Maybe many of them take place in high school because (this is just a theory of mine, birthed five minutes ago) it's easier to relate to for EVERYONE. A large majority (almost everyone) of the people who read VNs have gone to school at some point in their lives. So obviously most people will relate to a school-setting more than, say, working at a fast-food resturant.
And for your second question: A lot of people like romance. In theory, EVERY creature on earth like romance (and by "romance", I am mostly referring to mating). And being in love releases endorfins, which is the body's very own homemade "feel-good" drug, which makes you subconsciously see romance as something positive. And when your product is about what many people like, you will reach a wider audience. Also with romance, it's easier to justify H-scenes. But these are just quickly made up theories of mine, and should be taken with a grain of salt.
Also, since I am a huge fan of excessively romantic stuff (but curiously, only japanese romantic stories. it probably has something to do with the tropes used...) I would easily pass up on VNs not containing any romance...