In past discussions on the listserve, it is because: 1) it is a lot of work to make a Mac version, and 2) Mac is still a very small percentage of the total market in the world.
Many of us with Macs (myself included) use either Parallels or Crossover to run MLO on our Macs. I have Parallels open 100% of the time on my Macs, mostly for MLO. I certainly wish there were a Mac version, and I stopped using MLO for about 2 years because of no Mac version. But after trying at least 10 different todo list programs written for Mac, I came back to MLO and will never leave again.