True, it'd be ideal, but Mac and Linux both offer radically different ways of thinking, in terms of programming.
It would sap far too many resources, that we need to develop a version on any OS! Besides, on Mac and Linux, you can grab Virtual Box and load a Windows OS on there.
I don't honestly think you quite understand how much work that would actually be. First, let's consider the resources required to run. For Valve, this is easier, because they had direct access to the Half Life source code. However, the resources that the ESF Team need would be just nigh of impossible to get on Windows. E.g, the C++ Redist package. That would indeed be difficult to find a way to get around it. However all Valve needs to do is to grab the resources they need and wrap it up into a neat little DLL that only they can reasonably access.
Next, let's consider how much effort would be put in. Like I said, Valve have to do relatively little work in order to get a working Mac/Linux port. The ESF Team would first have to attempt to decipher considerable amounts of machine code and what not, and for what purpose?
It would be worth it to get a Linux build, only because Linux is useful for servers. For Mac - I don't think so.
, well a cheaper way on getting a OS X System is a Hackintosh but if it really need i could donate towards this as i am rather keen on having esf on OS X as i really only use my PC for websurfing, and i understand the reasons that the ESF-Team won't probably do this is spending another $699 on a machine and relearning everything for OS X but i can tell it would be worth the effort in the end 