The reason is because developing for the MacAppStore is actually a very different beast than for Steam/other DRM.
When we or the fine folks at Feral decide to do a title on the App Store, we in many ways are developing a different game than the Steam version. This of course takes a bunch of time and a bunch of money that many developers/publishers (especially smaller ones) decide not to pursue.
We did a great
blog article on the complexities of multiplayer development on the two platforms a ways back. It touches a bit on your question and is totally worth the time to read.
Outstanding article. The following is not a challenge to your expertise, just my observations.
It seems to me that what you see at the Mac App Store tends to be in a different realm, super casual gaming, mostly things like Plants vs Zombies, Simpson's Tapped Out, and Solitaire. This is not my defense of Apple, but an observation that when they take an iOS game under their wing, they are introducing it into their realm, their safety standards and right now they can afford to do that.
Myself, I'd never look to iOS to play any kind of a serious game, especially multiplayer. Although it could be fun on a local network to play some real time strategy games, like Kingdom Rush. But when I see a game like this, its simplicity seems to be designed for something like iOS, -casual-. For more demanding games, I'd break out my Mac, PC, or game console which are better equipped for such things. My uninformed impression is that for the types of casual games I would expect to play on iOS, developers can handle whatever Apple's requirements are, although that could be an issue for multiplayer.
If you were to categorize the areas where Apple sticks their nose into the game development of an iOS game, is it more than the control of adult content and DRM? Thanks!
