Hobo knives are heavy and unhygienic compared to a set of modern camping cutlery, I guess that's some of the reason. Also, a nice hobo knife will be more expensive.
I know both Böker (in their cheapo, made in China, line) and Case still have them in their line-up, though.
Case hobo knife, made in the USA: About USD 90.
Titanium spork: About USD 10.
I suspect that's the answer right there.