Because the Western Rome Empire collapsed under the weight of the sudden influx of the Germanic tribes, which were pushed in from the east beyond the Empire’s boundaries on the Rhine, the huge migrations were triggered by changing weather patterns in the 3rd and 4th centuries. Moving in an incremental fashion, the various tribes on the Eurasian steppes (band of the flat land that stretches from modern-day Mongolia to its western terminus in Ukraine) started moving west, seeking better pastures. This started a domino effect whereby the western movement of the tribes started squeezing the other tribes who lived western to the migrating tribes. Eventually, the Germanic tribes were squeezed more and more towards Rome’s boundaries on the Rhine.
As the Huns pressed West and disturbed the societal arrangement, it began pressing more Germanic tribes towards Rome.
In a few cases, the Romans allowed certain tribes to settle in Roman realms, like when Emperor Maximian allowed the coalition of Germanic tribes to settle in the province of Germania Inferior, in 288 AD after defeating them. But in most of the cases, the Romans resisted the sudden influx of newer people, because the migrations of newer peoples were occurring at a much faster rate than they were being assimilated into the fabric of the Roman identity.
Eventually, the push of the tribes from the east beyond the Rhine was so great that in 406, a coalition of Vandals, Suebi, and Alans, managed to break through Roman defenses and cross the Rhine. This was likely received as a shock both on the physical elements of the Empire, as well as on the shared consciousness of the Roman identity, for the Rhine was considered to be virtually impenetrable, and Gallic lands had been effectively Romanized for almost four centuries till the crossing. (For those more interested in this – read St Jerome of Bethlehem’s letter from the 5th century detailing the crossing of the Rhine and the ravaging of Mainz and other frontier Roman cities along the eastern boundaries http://www.documentacatholicaomnia.eu/03d/0347-0420,_Hieronymus,_Epistolae_[Schaff],_EN.pdf)
As the Germanic tribes ransacked Gaul almost in an unchecked fashion, in doing so, they greatly disturbed the pre-existing complex networks of political, economic, and social linkages that weaved the Empire together. So it was quite natural that as the Vandals appeared between Britain and Rome, Britain fell out of Rome’s hegemony, which in turn led to the appearance to domestic contentions to rule over Britain, which themselves didn’t last though, because even Britain became the subject of numerous waves of migrations (or invasions from the perspectives of pre-existing Roman Celts), by the Angles and the Saxons.
Any possibility that the technological know-how, the architectural influences and political frameworks from Romanized Britain would have been appropriated by the contenders that would succeed Roman dominion in Britain were rendered moot because the huge influx of migrations disrupted the conditions to such an extent that it led to a very real and sudden fall in living standards, quality of life, etc. The world quite literally collapsed for the Romans that were living in Britain in the early 400s.
Here’s Oxford historian/archaeologist Bryan Ward-Perkins explaining in detail the archaeological evidence displaying this sudden collapse of the Roman world in Britain.
What extended the Roman hegemony to all the lands in her vast realms were the complex networks that meshed with each other. These networks were disturbed as the Germanic and Hunnic tribes moved into the West, which cut off Britain from Rome’s dominion.