There were several reasons for steel.
One is that steel has become somewhat sidelined in weight-weeniedom, so it was an opportunity to bring it back into contention. You can easily calculate that by moving all the parts onto a CR-1 or 2006 Ghisallo, the bike goes from 5.5kg to something like 4.7kg (Tempting? - yes. Likely? Not unless I win the lottery)
It was an interesting joint project for Roberts Cycles and myself - I've had several of their frames over the years and I consider their work to be superb. I'm less convinced of Ti/carbon framebuilders' ability to provide a full custom service and a frame perfectly tuned to the individual at an affordable price. The ULT Roberts frame was totally made-to-measure with tubing choices to suit my purpose. (Again, maybe when I win the lottery, I'll set out to overturn this bias against custom Ti/carbon).
The frame was never meant to be the lightest (being steel, how could it be?), but I did do some research with S&S machine about the suitability of different materials for their BTCs before embarking on the project. Carbon is a no-no (though Calfee use BTCs on Ti tubes bonded to the carbon), Ti needs to have fairly substantial tube thicknesses, so only plain gauge Ti frames suitable (so no BTCs on a Ghisallo...), whereas with steel any of the lightest tubesets would still work OK, giving full rein to customisation of the tubeset for best ride quality. As it happens, the frame could have been a bit lighter, but for a problem lining up a different (lighter) carbon rear end with the frame geometry in this case. I've never quite managed to figure out to my satisfaction what the BTCs add to the frame weight - somewhere around 200g, depending on the weight of tubing removed...
The new Ritchey Ti/Carbon breakaway is listed as 1520g in 58cm size. Notwithstanding that the official weights nearly always turn out to be underestimates, it's almost £2000 for a stock frame, fork, headset and cable splitters, versus around £1600 for my full-custom equivalent (frame, fork, headset, BTCs, cable splitters, paint job) - and since my fork is almost 100g lighter than theirs, that's an equivalent weight with a lot more for less money...
Also, the beauty of steel is that it is repairable - maybe not easily in the case of my ULT frame, but it's nice to know that the investment is likely to last a lifetime (with the kind of mileage I'm likely to do on it!). In truth, if I could have a 2006 Ghisallo with BTCs, I'd take it. I love the ride of Ti best of all (fond memories of my Merlin Extralight - my first weight weenie project). I might even go for the Ghisallo without the couplings, but I would need to be rich(er)!