As far as I know there are at least two reasons for that:
* First, sensitivity of non-silver based emulsions is very poor, good for
manipulation purposes, but forcing you to very long exposure times if you
don't use very strong light sources (reciprocity effect produces almost
geometric progression)
* Second, most of the emultions are only sensitive to UV light, giving to
your enlarger lenses the chance to block most of the light coming out of
there
So, these are the main reasons (maybe there are many others...) that are
giving poor chances to us to use standard negatives and the well beloved
enlarger.
Regards, Andrea.