Depending on heat treat, carbon steel can be tougher (bends rather than breaks) than stainless, and take a finer edge, stainless can hold edge slightly longer as it has a higher abrasion resistance, and is more resistant to corrosion, sacrificing sharpness due to large carbides formed during heat treat, and can be more brittle at the the same hardness as a simple carbon steel. This means that during use the edge wears unevenly and forms micro serrations around these carbides, giving a toothy, less than razors edge to the knife. However, Sandvik 12c27 is a very finely grained steel, so with a good heat treat has a good compromise of attributes of both carbon and stainless steels.
Mora stainless blades are usually polished, which can heat up the surface enough to temper a few microns of steel and make the edge softer and more prone to micro chipping, which may be why people mention about the stainless not holding its edge, usually a sharpening session will sort this out.
These aren't definitive reasons, just information I have picked up over the years.