The answer is: it didn't.

Great migration periods result from periods of great growth in population. One is beginning in Europe as we speak and if you consider that the official prognoses envision as many as four billion people in Africa by 2100 it will only get more intense.

The 20th century was a period of great de-population but before the was a great migration period which involved the population of Europe migrating out of Europe as a result of the huge increase of population during the industrial revolution.

Before then you have a series of bloody religious wars and the Black Death to manage the population and shortly before then you have the period in which Slavic tribes became sedentary and got involved in contact with the Germanic tribes which settled five centuries before. In that time you have what can also be understood as a migration of sorts - the Mongol conquests.

Just because history doesn't classify it as "Great Migration" doesn't mean that it doesn't take place. Definitely the growth of human settlement in Europe helps to muddle the clarity of the picture. In Roman times you would have whole wandering tribes going through wilderness and as such those temporary settlements would function as indicators of their journey. After Europe became settled it is much more difficult to gauge the size of the actual migrations but they take place all the time.