It's just cost. A good example is Minolta. Soon after they started making auto-focus SLRs & lenses in the mid 1980's, they moved all of their manual focus SLR & lens production to Seagull plants in China. (I assume much of their rangefinder camera production moved to China as well). Others did the same thing at the same time, such as Yashica -- and probably Cosina, too. Other companies tried Malaysia, Taiwan, South Korea, etc.
When Minolta stopped making manual-focus SLRs, Seagull continued to make them, and sold them under the Seagull, and several other, labels -- from Argus to Zenit -- and they added a lot of nice features that Minolta SLRs never had. How about vertical, metal, mechanical SLR shutters with 1/2,000 shutter speeds? No battery needed!