IMHO success and failure always fall on those that lead the endeavor.
I think there was many reasons why it did not sell well, most reasons were out of the devs & publishers hands.
Word of mouth/hype, gaming press, competing with morrowind & mainstream mentality. Deus ex has an intellectual title and it sold well enough.
I blame the audience mostly. I had
never heard of any of my now favourite games whilst growing up, I bought deus ex only because it was cheap and after that I had to hunt the rest down on the net. And i tell you now they only get better with age, especially when you compare them with the mostly mindless crap that is on the market today...but i suppose that makes them even more special.