Sigma’s R&D has not developed new DSLRs fast enough to keep up with market demands and the wonderful capabilities of the Foveon sensor are not put to proper use.
The Foveon sensor is remarkable in that it captures RGB color at each pixel due to its three plates (vs. a single plate in regular sensors). It is supposed to give much more accurate color reproduction than regular sensors.
Unfortunately, because Sigma has not worked fast enough to create DSLRs that can truly compete with those made by more popular camera makers such as Canon and Nikon at all leves (including, but not limited to low light performance and HD video), its DSLR arm now finds itself in a terrible slump.
Their latest offerings, the SD15 and the SD1 have not sold well, unsurprisingly, and I hope they do something amazing soon in order to catch up with consumer expectations.