One big question to which I’d like to know some good answers but which I hardly have the time or training to pursue in any great depth concerns the Germanic migration period (“Völkerwanderung”) in the post-Roman world. Though there are multiple facets to it, perhaps it’s best expressed as the question of why Germanic languages and cultures were lost in most places as the new military rulers were absorbed into local populations except in Britain, where the composite Anglo-Saxon-Jute culture and language(s) replaced local cultures and languages.
The answer previously given was very succinct: the invaders replaced local populations, so it’s no surprise that their language(s) and customs prevailed. Germanic invaders in other places were absorbed and their cultures disappeared because they were a small percentage of local populations and did not replace them. The problem is that scholars generally no longer believe that this Anglo-Saxon replacement happened. Archaeological and genetic studies paint the picture of a fairly small Germanic population transfer and admixture into a stable population base that continued throughout the period, as in other former Roman territories.
The economist in me wants to connect it to trade networks. The Franks, Goths, Vandals, Burgundians, Langobards, etc. that took over various patches of continental Europe and Africa were numerically inferior to the populations they oversaw just as the Anglo-Saxons were. But they were well exposed to Roman culture and to a large degree were part of it. As their remaining legal texts demonstrate, their law was much less developed than Roman law. As a consequence, for most things they simply continued the use of Roman law. This Roman law evolved to deal with the complex society and economy of the Romans, including in large part economic phenomena; land ownership, trade, etc. were very common topics in Roman law. The Franks, for example, took over military jurisdiction in northern Gaul but had no system even remotely comparable to the trade and production already happening there and knew the goose that laid the golden eggs when they saw it. The material culture they merged into was far more advanced than theirs and they naturally wanted to be a part of it. Markets existed that connected one town to the next, to the next, to the next, all the way to Rome, Constantinople, Antioch, and Alexandria. It was a simple thing to keep using Roman roads and waterways.
The post-Roman Britons were already on the fringe, and it seems that they were not particularly thoroughly Romanized outside of the elites. When Roman forces withdrew in 410 trade no doubt continued, but being on the fringe and losing this direct connection with the continent, their trade must have suffered. One of the classic examples used when discussing post-Roman Britain is that they had a thriving brick industry that steadily declined, and when it finally died no native brick industry replaced it until centuries later. Another example is that they used pottery wheels when connected to Rome, but this practice too fell into disuse. Capital deepening allows for greater productivity, but is not necessary when this productivity goes to waste. As the division of labor is limited by the extent of the market, we can reasonably infer from these observed phenomena that their markets shrank. It’s possible that native Britons were not the brickmakers and potters in Britain, but the fact that these practices faded away after the Roman withdrawal rather than stopping cold makes that implausible.
Britain was also at the fringe in terms of Church influence. While the Church was not yet the centralized structure that we see in the High Middle Ages and beyond, there were centralizing forces at play, the influence of the Roman government first and foremost. And viewing the church in its social aspect and not its theological aspect, this also puts Britain on the fringe of European culture of which church and market were components. Trade is certainly possible in these circumstances, but it less likely than it would have been for Gaul, Hispania, etc.
As an aside, implicit in what I’ve said so far is a general acceptance of Henri Pirenne’s thesis that “classical” Roman civilization ended in the eighth century when the Mediterranean was lost to Muslim cultures, destroying the trade networks that had sustained Romanitas for so long. As the extent of the market shrank incredibly, it’s no wonder that the cascading effects would greatly reduce material prosperity for the average European and consequently shrink things that benefit from material prosperity like higher learning, literature, art, and the other “finer things in life”. This process may have been started by the division of the empire into distinct legal jurisdictions under Germanic rulers, but the loss of the Mediterranean was a crushing blow (for the Europeans, that is; the newly-Muslim lands prospered).
Back to the matter at hand, this diminution of cultural contact and especially of trade with the Continent would have made the Britons much more susceptible to cultural takeover by the Anglo-Saxons. If, as we now believe, the Anglo-Saxons who went to Britain were not entire nations but only a small number of settlers, it’s likely that they would have maintained trade connections with the rest of the Angles, Saxons, and Jutes back in their ancestral homelands. The way to reestablish more extensive market connections and regain material prosperity would have been to go along with the Anglo-Saxons rather than cling to their fading Romanitas. And we know for certain that the Britons did go along with the Anglo-Saxons. Add to this the fair presumption that the Anglo-Saxons were rather more united militarily than the Britons, which would have made it easier to make their political elites focal for the island, and we may just have ourselves a decent explanation.
While this all seems reasonable to me and fits with my base of economic theory, there are, of course, a million and one other factors that could have been and probably were important. As most historians whose sources I have access to are not economically-inclined, they never consider the issue this way, or if they do I haven’t discovered them.
My final note is about taxes and tariffs. I know that the Romans had trade restrictions of some kind on some outsiders—many of the treaties with the Goths included removing or reducing trade restrictions, for example—so it’s possible that the post-Roman Britons suffered from a lack of free trade with the Romans after the Roman departure, making the Anglo-Saxons doubly appealing. But given that it had been part of the empire, it’s also possible that trade restrictions would not have been implemented against British goods after the withdrawal. This I have no idea about, and again, no time and training to find out about.