Chester's Fort on Hadrian's Wall

A cold visit to Hadrian's Wall. I got the feeling I was at this same site 20 years ago. It's certainly possible. The Romans knew how to pick 'em--Chester's Fort is on a high bluff overlooking the North Tyne and the bridge they built to cross it. Anyone know why the Romans abandoned Hadrian's Wall, and, ultimately, Britain, leaving the remaining soldiers to melt into the surrounding countryside and become Brits? Or why they thought Britain was worth conquering in the first place? It was a long way from home, the resources were not obvious, and the tribute extracted from the subject population probably did not equal the cost of maintaining garrisons across the island. But maybe the Romans, like the rest of us, were seduced by England's green and pleasant land.