Fairchild rationalized that since the guy in the back wasn't flying the aircraft, the titanium tub wouldn't have to be enlarged to protect him. Some people in the West German contingent thought that was BS. Probably the back-seaters.
That would also be anyone who had anything to do with the A-10’s spiritual progenitor, the IL-2 Shturmovik. Early two seat IL-2s had the same idea - armor for pilot, none for backseater. Let’s just say this turned out to be a horrible idea and that nobody who saw the results was at all inclined to try that again. And in the 70s, there were still people in the West German government who had fought on the Eastern Front of the ETO who had seen the results of that idea first hand. Neither the Russians nor Germans were interested in repeating that.
But yes, titanium is a fungible material these days, if a bit expensive - but most of the expense these days is due to how hard the stuff is to work and form and not rarity of availability. I mean, they even make gearshift knobs and, er, ‘adult toys’ from the stuff now.