Further to Lurkers contribution, the Clutha building was originally a multi-storey that was partially demolished. This is seen in many places across the city where clearance has taken place leaving the ground-floor pub as the only remnant of a block of tenaments. Hence the walls are pretty substantial but the roof is much newer and flimsier than the apparent age of the building would suggest. No surprise that it couldn't hold a 3ton chopper even if an attempted autorotation had removed most of the energy.
Given the variety of more benign landing spots available - King St car park, the Briggait Broomielaw Stockwell St intersection, Clydeside Walkway and the Clyde itself, I'd go along with the supposition that the pilot flared out of the autorotation too early and landed on the pub, not the intersection.
Doesn't explain eyewitness statements about tumbling end over end though.