I think there are many reasons (most of which have been outlined here) but additionally to those like me who started photography in the 1970s or before, "photography" was usually associated in some way with Kodak, Ilford, Agfa and a few others. Whatever brand of camera/lens you might buy, you'd probably use one of their products to shoot or print on. All of a sudden, the advent of digital cameras meant that photography became the domain of Sony, HP, Samsung, Panasonic and a wealth of others plus the computer and software people. Many of the results of these never got as far as being printed, so the materials market probably shrunk dramatically (my guess). Ilford and Agfa hit the doldrums some time ago and Kodak are a case of 'the bigger they are, the harder they fall'. I find it hard to see how Kodak could ever have competed when the photography market was suddenly thrown wide open to so many electronic giants. Perhaps if they'd realised the potential of the digital still camera they'd invented much earlier and invested a huge amount of money into patent protected development, they might have stolen a march on their competitors, but I suspect that would have been temporary. Perhaps diversification would have been the answer, as with Fuji, so that we could now be tucking into bags of Kodak Crisps (OK, 'chips' to those in the US).
Steve