Macs are popular but they make up only 3% of Steam users. As a solo developer it is hard to justify spending time on a Mac version which would have a limited audience, plus the fact that (unlike for other platforms) you can only create a Mac version using a Mac, which means I need to buy the hardware, do the porting work and then keep it synced with my Windows files for every update I release. That isn't a small amount of work I'm afraid.