Well, if you think about when the Six series were made (1970-1988) fiber optics basically weren't heard of and the Six series were just starting to compete with the S&W's and Colts. Sight manufactures like Millet did make some replacement sights for the Sixes, but compared to the Smiths and Colts the Six audience wasn't real big so they didn't go after that market as they did the other guns.
Then the GP100's and Redhawks were coming out, with the easily replaceable front sights, and fiber optics were beginning to be more main stream and with the Sixes being discontinued, there wasn't much reason for them to market sights for them.