Default Why can't port proprietary from GE to CM11 and such?

As we know our i9505 has Google Play Edition, which is pure AOSP.

And the ROM like CM11,PA and other AOSP based ROM always have some problem with hardware here or there (like camera? )

So if we have GE ROM and it's AOSP, why can't port it to CM11 or something to make the ROM better?
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GT-i9023 Nexus S
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