I think pretty much because it was derailed by Atari over DRM issues. Mysteries of Westgate was a decent enough module at a $10, but it was completed in 2007 and then only released in 2009. I think that heel dragging by Atari may have scared off other potential commercial mod builders. The fact that the fan-built Baldur's Gate Reloaded module has been downloaded (uniquely) 25,000 times and IWD 9,000 times shows there's still a market, albeit a small one.