Spiderman is easier because he has a bunch of classical issues that you can make drama out of. Superman either has to contend with kryptonite (done a thousand times before, good luck being original), or needs to have some psychological trouble to deal with (very difficult to do right in a superhero movie, especially without diminishing the character). Superman's just too powerful.

I suppose they could adapt the most popular comic arcs, but something new would be exceedingly difficult.