It's probably just that the Bernette machines are repackaged Janome designs (as I understand it). The B77, and the Janome it's based on, were probably designed with the dual feed as an original feature.
I imagine adding it to a completely different machine (ie, an other Bernina) is likely prohibitive. Either from an engineering perspective, or a patent licensing perspective if Pfaff have a patent on the system.
bm