I've noticed the same when looking at old Georgian and Victorian maps of London. You get these surprisingly sharp edges between urban and rural. You often have streets lined with quite grand buildings and nothing but fields behind them. It's quite strange when you're used to modern cities that gradually peter out into suburbs.

My guess is it's because at this point the population of cities was growing quickly, but the large scale migration of farm laborers into them hadn't begun in earnest yet. So most of the housing being built at the edges was intended for the expanding merchant classes, who wanted something a bit more impressive, and who also had live in servants. The Georgian terraces of London are typically three or four storeys, with the top storey being rooms with low-ceilings where the servants lived.