The Roman historian Walter Scheidel has a whole book (Escape from Rome) on this question -- basically, exploring the idea that Europe never really reformed a la

The Roman historian Walter Scheidel has a whole book (Escape from Rome) on this question -- basically, exploring the idea that Europe never really reformed a large-scale empire after the Roman Empire started to fall apart in the 5th C, whereas other parts of Eurasia (esp. China) did go through periods of reforming large-scale empires, sometimes larger than past empires in the same region. Unfortunately, I'm only about halfway through it, so I haven't gotten to the part where he tries to explain why this happened. But so far, it's quite good.