1) Macs are harder to consider secure, they're effectively all jailbroken. Cheating/bypassing in app purchases will be rampant, reducing the opportunity for cross play, and the Mac market isn't big enough itself. These aren't insurmountable issues, but they require investment, and the additional Mac market probably isn't worth the outlay and risk.

2) You have to rebuild the UI, which costs money which the Mac version may well not recoup.

3) You have a different version for desktops that costs more upfront with less reliance on in-app mechanics that you don't want to undermine.