To understand "why" one has to understand basic differences between b/w films and color films. B/W films have always been designed to "work" with conventional b/w chemicals. These basic developing and fixing agents were standardized a century ago, and even though there are infinite variations they still work in basically the same way. The developer converts the exposed silver haliade to metallic silver in proportion to the films exposure to light, and the fix removes all the undeveloped silver. This concept has never changed.
Color films, on the other hand have been re-engineered from scratch many times. New dyes were developed over time, new color couplers, etc. It has not been possible to make newer, better, improved color films with better dye stability compatible with older processes. It has also not been possible to make older color films compatible with current processes. Until the introduction of Kodaks C-41 negative process and E-6 color slide process, all manufacturers had their own unique color film processes. It is not just Old Kodachrome that cannot be processed in current Kodachrome chemistry, but in essence ALL color films prior to the current color film processes cannot be processed for optimum results, or in most cases any results. Since color negative and Ektachrome (type) processes are vastly more simple in processing than Kodachrome, some custom labs are willing (for a price) to custom mix and keep chemistry to process the older consumer films made prior to C-41 and E-6. Kodachrome is a different beast. It is probably the most complex color film process ever engineered. Doing small quantity processing, ever so often, of obsolete Kodachrome has never been economically feasable. Also, the older color negative and color slide processes (other than Kodachrome) can be done in simple tanks with inversion agitation, and no big investment in specialized equipment. Kodachrome, on the other hand has always required very specialized automated processing equipment to process, and these machines are all dismanteled and mostly scrapped.