This is an extension of this question. As noted by that question, Apple discontinued support for many of their older computers, notably the plastic MacBooks. What is the reasoning for leaving out this hardware? Is it due to hardware restrictions or is it just a push for owners of older hardware to upgrade?
The limitation is because Mountain Lion dropped support for 32 bit kernel and extensions. There are work arounds that work on some old computers (like the first gen Intel Mac Pro), leading me to believe it's simply an artificial limitation, much like the exclusion of Intel Core Solo support that came with Lion.