The components might meet Android 4 requirements but you often don't have the info needed to write drivers for them
Typically for lower end parts the makers implemented the simplest cheapest thing they could get away with - so buggy limited and out of date drivers - and they have bought in modules from others, so nobody has the complete specs.
For better system the code in the SoC (and GPU especially) is what distinguishes it from other models and competitors so they are very careful about who they give details out to. Otherwise you could buy their $1 SoC and turn on the features in the $5 part.
If you are a Samsung or an Apple you can control the supply chain better and insist on having the info and compliant drivers - but most of these are aimed at mobile phone makers who rely on you having to buy a new model every 18months rather than doing updates.
Cyanogen etc do a very good job of custom kernels but they can rarely insert a much newer driver into an older chipset.
I've never really seen a clear answer. Whatever the Cyanogen Mod team is up to, it must be damned tricky.