There is lot of misconception about the endurance limit of steels. The performance is pretty impressive on a properly prepared lab sample. Unfortunately, a bicycle frame is far from that and variations in manufacturing processes can cause fatigue failures at well below expected loads.

It can argued that a higher end frame with a good grade tubeset is less likely to suffer fatigue failure because more highly skilled craftsmen typically build these frames and take greater care in the construction. Still, I've seen lots of fatigue failures on frames of all levels.