Why keep developing for Snow Leopard in 2015?

Although Snow Leopard is an old system, it is still running in 1 of each 5 of Apple computers, mostly workstations.

Those who spent in expensive software and hardware and invested time in developing and learning a workflow may want to keep their system running.

If everything works smoothly there is a natural resistance to change, change that is mostly pushed by a programmed obsolescence rather than real innovation. Why upgrade to do the same thing, with the unknown of what else will cost the upgrade in terms of compatibility and without improvements in efficiency?

If you or your company is developing software for Apple Mac OS, it is sensible to include compatibility with 10.6 Snow leopard. It will cost you a bit more to develop for an outdated system, but it can pay off, as your company could be the only one in the market offering a solution. Therefore you can charge extra for this service.