Well for starters, some people might want to respect copyrights, especially for open source projects)
I take that back, they already ripped the entrance and exit graphics, so I don't think that's their concern.
Maybe they're just too lazy to rip the other graphics?
It's quite possible that this is a still-in-progress project, so they just don't have time to finish all the graphics yet. Or this could be an abandoned project where they never got to the point of finalizing the graphics.
Sourceforge projects are generally open source, so feel free to contact the programmer(s)/development team to see if they're interested in obtaining rips of the original game graphics. (Or heck, even just sending them the graphics in default.sty would be an improvement.) This is also a good way to check whether the project has been abandoned (if no one respond, then obviously it is abandoned).
Aside from the bad graphics though, how's the gameplay?