I really think quilter was going for a "system" more than a stand alone amp -- quilter amps and cabs that can be scaled from small to as big as you could possibly want. My feeling is that lead to several design choices. We have confirmation from quilter that the 1/4" outs were part of a design choice to make things work with their cabs, although we know that many players want speakon out to use with their non-Quilter cabs. When you think of it in those terms, and that so much of Quilters stuff (on the guitar side of the house) is geared for "pedal platforms", including the slave out rather than a DI out would make sense, assuming that bass players are also using pedal boards, with DI/pre amps on them.
I would guess maybe they did not anticipate the BB becoming such a popular stand alone amp. Also, I've been in some "high end" preamp threads lately, and a LOT of dudes are picking the quilter as their power amp of choice.
All total speculation on my part, of course. A rep from quilter was on here a while back, and got a lot of feedback from the community about what features they would like to see. The DI and speakons seemed to be on everyones list.