I'd conjecture the main reason for rolling threads would be to make the threaded portion of the steer tube slightly stronger in case someone adjusts their quill stem too high. Of course that's not supposed to happen - but it does. A second possible reason might be because it's faster. I'm sure the tooling cost plays into it as well.

AFA a mandrel, yeah it would be needed, but you've got to assume tubing factories are mandrel equipped...

I seem to remember seeing rolled threads on old mid level mtbs and other bikes, but frankly it's been so long since I've worked in bike shops I can't say really. In the previous discussion I'd just assumed they'd be rolled because it's faster and stronger, but clearly that is not correct, at least for higher end steerers.