It's popular in the instance of this IPO because the underlying asset is on BitFunder and the other two exchanges will serve as pass throughs.
As to *why* they chose BitFunder, I don't know. It's a piece of crap compared to BTCT and Havelock. BTCT really takes the cake I would say.
I believe it was mentioned that all the 'virtual' talk on BTCT served as a deterrent.