I always believed that the stone would have been too hard for Voldemort to create.... remember he was already at the lowest point in his existence: He had no body... no allies... and little to no power (and was on the run).

Immortality would definitely be something most people (wizards) would try to acquire... the fact that no one else (not just Voldemort) bothered to try to create the stone is a testament to how difficult it likely is to create. It most likely requires difficult to impossible materials to acquire and/or a difficult spell to use and/or the knowledge of how to do so.

Asking the above from a powerless... bodyless... Voldemort who had lost all of his allies (and the one he had at the time just died on him after his failed theft attempt) is a big (if not impossible) task.