Is it because programmers are more willing to help people?
People come to SU when they have a problem. When the problem is solved, they stop coming. That's not the case on SO, where people also want to learn, to perfect themselves. When they help people they also learn from others. This is a big difference from SU, which is viewed as nothing more than tech support, unfortunately.
A second major factor is that SO has a much larger userbase, almost 10 times larger than SU.
For one thing. it's almost one whole year older. SU launched on Aug 18, 2009; SO launched on Sep 16, 2008.
Stack Overflow launched with two fairly large programmer audiences from Coding Horror and Joel on Software, so it had a substantial head start as well.
Mostly, it will take time to grow the community. We're looking into a few different ways to help that happen.
That has nothing to do with it at all.
Stack Overflow has a direct audience; a definable audience that spends its life on the intertubez.
Normal people (and people who might need tech help) do not necessarily realize that Super User is even here. It'll take far longer for it to gain widespread adoption than it took Stack Overflow. I gather it'll have that widespread adoption, but only if it is very well maintained, and if people treat it like they do Stack Overflow and edit posts; fix things, and generally don't leave Broken windows lying around.
SO users are 99.99% developers. Developers helping each other, nice! A search on Google brings you to SO easily.
SU users are mixture of all sorts, and people who had questions do not have idea that SU exists. A search (if done properly in the first place) probably give them a lot whole other resources too.
If a user has network/bootup issue (very common) and can't get online, he/she ain't gonna ask on SU (unless using another terminal elsewhere). WHen he/she is back online, problem solved, nothing to ask. :P
I think that of all computer users, it's the software developers who are looking for answers on the Internet. Regular users first read the manual, the helpfiles, ask a colleague, friend or anyone else. Developers will first go to a forum, ask questions and in the meanwhile they're trying to solve the problem in some other way or work on somthing diofferent first.
The difference is that developers will freely share their knowledge with others, and that most development is very generic. Superusers are often asking more specialized questions and thus asking on a large, generic forum isn't really helpful. For example:
When I ask something about writing assembly code in Delphi, the question would be a bit obscure but I have no doubt that I will get 50+ views with at least 3 answers. It's very likely that one of those would be the right one too. When I would ask about how to import a Collada model into Poser Pro, I do have more doubts about getting a few views and probably no good answers. (Okay, some will just google for Collada and Poser Pro, hoping they can find an answer and thus gain reputation thus it's the reputation hunters who will most likely answer this Poser question.)