The Roman army was overstretched by the time they got to Scotland. They committed a disproportionate number of legions to Britain, unlike Germany where they wihdrew, or Dacia (Romania) where they wiped out the native people and planted a friendly population. It's probably significant that the British were the only Romanised Celts to retain their language rather than adopting Latin. Even the English with a millenium longer haven't managed to erradicate British.