This isn't rocket science here. These motors are just old, not very plentiful, not parts compatible with any of the other B-series engines, and there's just really nothing outstanding about them (no VTEC, no ability to swap heads with other B-series, etc.). They're not awful motors, but there's nothing really awesome about them either.
Given all the things I listed, it really makes sense that these engines don't get much love from the after-market. Why would the parts companies go gung-ho on a mediocre series of engines that have been out of production almost twenty years when there are newer and much more tuner-friendly motors like the B16s, B18s, H22s, and the K-series?