One big factor for single-bore subway is that can fit the stations inside the tunnel. That saves a lot of money digging out the stations. I think Barcelone Lin

One big factor for single-bore subway is that can fit the stations inside the tunnel. That saves a lot of money digging out the stations. I think Barcelone Line 9 is the one that popularized it. I suspect the other factor is that they figured out how to make large TBM.