They didn’t have time, really. The Stalin Line started out construction in the late 1920s, and was a series of fortifications meant to disrupt an attack from the West. It differs a little from the Maginot line as the French couldn’t afford to lose any ground (as that ground contained a large amount of their industry) and the Stalin Line was more for breaking up an attack to make it easier to counter, but was similar in that it was a series of well constructed fortifications with large guns. A remnant of the Stalin Line is still open to visitors in Belarus, about 45 minutes West of Minsk.

However, once a border started to exist with Germany (German occupied Poland, but who’s counting?) the Stalin Line was a little bit too far East. So, it was deactivated and the Molotov line started.

Those cunning Germans, though, attacked before the Molotov Line was finished, but the Stalin Line had already been decomissioned and wasn’t of much use either. So, in the end, really it was just mobile forces which were available to deal with the Germans