It didn't end abruptly. In between Aristotle and the enlightenment we had enormous strides in the field of mathematics. Everything from negative numbers to: algebra, trigonometry, and calculus was developed in the period between Aristotle and the Enlightenment; heck even the concept of zero as a number comes from the 9th century. During that period of time we have such illustrious minds as: Euclid, Archimedes, Galileo, Kepler, Copernicus, Descartes, and Pascal. We also have the inventions of: glass blowing, steam engines, the astrolabe, turbines, the printing press, the telescope, the compass, and even the thermometer. Entire fields of study were developed during this period of time as natural philosophy gave way to physics, alchemy to chemistry, and cell theory spawned Biology. Modern universities were first established during that period of time as scholasticism began to take over in the early middle ages. Heck, even the modern scientific method itself was pioneered by Roger Bacon at Oxford university in the early middle ages (which led directly to galileo, Copernicus, et al. 's discoveries).
So, perhaps a better question than why succession of knowledge abruptly ended for fifteen hundred years (which, it didn't); is why there are otherwise intelligent people on this world who are that ignorant of history.
