Originally posted by i5_david it is only a change of the little tiny metal element on the back end (I am sure I am missing something, so I fasten my seatbelt to learn something )
Okay, I'll bite.
Sigma's mount change program only applies to lenses that they already produced in K-mount, meaning they produced enough spare K-mount-specific parts for that lens during the production run to be able to switch out those parts in the service center.
Every version of a lens requires a certain number of parts that are custom for that version (of course there's more than just the metal mount plate - the aperture control mechanism is different on each brand, as are the electronics). The lens maker is going to do their best to reuse parts in multiple designs, but there are still the labor costs of designing/testing the custom section for each version of a lens. If they have to make custom tooling for a version, well, precision tooling isn't cheap. In addition, there are fixed costs to stopping the assembly line to retool for a different product, whether it be a different version of a lens or a completely different lens. There are inventory costs of storing and keeping track of each version. It adds up.
Pentax is only about five percent of the DSLR market these days, so (if the proportions hold for lens sales) for every K-mount version of a lens sold, they sell ten Canon and nine Nikon. The lens maker has to decide if the fixed costs are worth it when they are only going to sell five hundred Pentax-version lenses, compared to five thousand Canon-version - or if they would be better off switching to another design that they can sell five thousand of. They could pass the costs on to us in the form of higher prices (and we'd complain), they could cut features like OIS and leave the price the same (and we'd complain), they could get Pentax to subsidize the higher costs and pass them on in higher prices for Pentax-branded versions (and we'd complain), or they could just say "it's not worth it".