Why were most old hollywood films shot on a studio set instead of on location? when did it start to switch?

Since questions are permitted here, i was wondering if anyone knows why many old black and white movies, like it's a wonderful life, were filmed in a big studio set? i'm not talking about flying to iceland, i'm talking about casual street scenes or neighborhoods. things that can easily be done on location. it's rare now to find a movie filmed in suburbia that doesn't actually use a real neighborhood.

so why the change? when did it change? was it simply convention?