The problem is, they really can't use any of the Windows-specific code (like what's used for CDs and whatnot which relies on Windows components) and they have to re-write the code from scratch for MC for Linux and Mac (though MC for Mac does support CDs). And they've been doing that over the last couple of years, slowly but surely.
However to be fair, what you said might've been somewhat true years ago when MC for Linux and MC for Mac were first released, but since then they've been reaching near feature parity with MC for Windows... or at least as close as possible to feature parity as possible (explained more below). Both MC for Linux and MC for Mac have matured quite nicely over with images support, video support, theater view support, etc. being added over the years. MC for Linux CD support I recall Bob saying would have to be rewritten from scratch, but he did mentioned he thought he could get it working faster by using cdparanoia (libcdio likely could be useful too). Hopefully that can come to fruition with MC26 for Linux.
Keep in mind, there are features that MC for Windows support that MC for Linux and Mac will likely never support. Things like madVR and LAV Filters (which rely on DirectShow, a Windows component) and quite possibly television support because of the lack of drivers on Linux and Mac. There's probably other things that escape me at the moment too however I vaguely seem to recall ISO mounting support might not be doable either (MC for Windows, again, relies on Windows components for this), but I could be wrong.
Bottom line, it'll gain CD support eventually, but that depends what else is higher on the ol' to-do list.