The Steelers defense didn't do a good job keeping the 49ers out of the end zone in the second half. I'd say it's because they were fatigued.
Here's the Steelers offensive drive chart from Sunday's game:
--4 plays, 5 yards, 2:00.
--3 plays, 5 yards, 1:26.
--6 plays, 16 yards, 2:40.
--4 plays, 15 yards, 1:45.
--3 plays, 1 yard, 0:48.
--11 plays, 34 yards, 5:55.
--4 plays, 7 yards, 1:41.
--3 plays, 5 yards, 1:08.
--3 plays, 82 yards, 1:31. TOUCHDOWN.
--3 plays, 9 yards, 1:55.
--2 plays, 81 yards, 0:56. TOUCHDOWN.
--3 plays, 8 yards, 1:21.
--4 plays, -2 yards, 0:38.
San Francisco held the ball for 36 minutes. The Steelers defense only gave up 3 points in the first half. They gave up three touchdowns in the second. Probably because they had to defend about 20 more plays than the 49ers did.