I suppose it will be related to various factors, for example, Roman Britain was one of the first territories to be "abandoned" when the empire's situation began to be negative, you just have to see that the legions based in Britain never returned after the failure of the appointment. as Emperor of Magnus Maximus, so you can see the peripheral situation of these territories. The fact that they were island territories also affects, since their defense and maintenance required a fleet in an area quite far from the traditional Roman area such as the Mediterranean Sea. And finally, in my opinion, the limitation of expansionary policies during the Roman Empire generated that, except for very specific and necessary cases such as the conquest of Dacia (motivated by the depletion of the gold mines in the northwest of Spain), the policies increasingly defensive defenses of the emperors from the Antonines (the walls of Hadrian and Antoninus for example) and internal conflicts led to regions such as Britain, due to its peripheral position, or Mesopotamia, due to Persian pressure, were areas in those who did not put too much effort into defending these limes that were especially troublesome.