Ubuntu doesn't say "too bad, we no longer own any six year old Thinkpad T60's" when they release a new version. They simply leave the drivers for such devices, make changes that aren't expected to break specific hardware models, and get some stablish prereleases so they can take the occasional bug report for the leaky abstractions that did break.
Contrast to Google who AFAIK throws globs of code over the wall with all of the support for older devices ripped out. And then CM presumably uses that tree as a starting point to make changes, instead of merging into a master CM tree.
CM might just be doing the best they can to cope with Google's broken process, but the whole philosophy of brittle trees tailored for specific devices is the philosophy of embedded development, not open source. So CM comes off as aiming to be the latest eye candy fluff mod for tinkerers, rather than a solid computing base for your phone.