There's plenty going on during the middle ages in theology. You've got Anselm, Albert, Aquinas and Dunns Scotus as the big luminaries, and there are a lot of "smaller" but still important names. I think there are a few reasons people shy away from this period in regards to popular interest and scholarship. First, there wasn't some big schism or some big controversy that spilled over into the legal, military, and economic lives of people like the Reformation/Counter Reformation. Second, it wasn't foundational like the Bible or other very early Christian writings. Finally, the middle age theologists were intentionally hard to understand. They used a very unique jargon and wrote their treatises in complicated logical formats. The entire point of this was to keep the arguments between on the most education people and weed out the laymen. So no surprise that laymen then or now have trouble getting into their writings