The number of people who are actually playing CloD is quite low. So low that the three main forums dedicated to CloD have the same 15 people posting in all of them, and little else.

Converting to Mac would take time and money. Even if they paid some other company to do it, there is still the money issue.

The number of Mac owners who would play this are probably countable on one hand.

It's just not worth it, hence there being almost no games for Mac.