I think it's because, in the world of programming, people are incredibly critical. Just try putting absolutely anything out there, and you'll get certain types of people picking it apart.
I remember an early experience in my working career, when someone was sharing their sample code with a group, to demonstrate a particular concept. And one of those present picked them up on their use of magic numbers, as if that was at all relevant in the context.
I don't blame anyone for being wary of showing their work in progress. Painters often don't like their subjects trying to take a sneek peak at their work in progress, as another example.