Hey, one of the co-founders here.
No particular reason - all four of us are web developers, and we quite simply, we each badly wanted a platform for web development screencasts to exist.
We differentiate ourself from YT in a few big ways. The first is our focus on developers. A YT Partner can post videos exclusively on development, but it's obviously impossible to include code markdown or have meaningful software-related tags. We also feature the ability to offer both free and paid videos; it takes a lot of time to make quality screencasts, and many of the developers we've spoken to have found YT insufficient for monetizing their content.