Big Marketing Names?

A lot of bottom brackets were largely hollow. Campagnolo Record/Nuovo Record square taper bottom brackets were hollow.

Octalink, and ISIS bottom brackets are hollow.

And, every 24mm and 30mm "two-piece" crankset with external bearings bottom bracket is hollow.

I think Shimano would have gotten into trouble had they called their Tiagra C-Channel cranks "Hollow". Plus they want something easy to differentiate their upper tier products from their lower tier products.