Presenting his work and then hear critics at an early stage might make him loose interest.
Well, some people can handle criticism, and some cannot. Either way, Jim has his reasons and we've grown to accept them because the fruits of his labor more than make it worth the wait.
Still, a LOT of software developers show screenshots of their work prior to the software going gold and even prior to beta versions, so it's not like it's taboo or something. The difference here is that Jim is in fact an artist and as such probably views things a bit differently than your average programmer.