They probably don't want to give Motorola a Nexus, at least this year, so the OEM's don't get scared and push Windows Phone harder or something. It will probably happen in a few years, but not yet.
Also, Motorola needs to make more worldwide phones. Samsung made the Galaxy S and S II, which earned then the Nexus S and Galaxy Nexus. LG made the Optimus G, which earned them the Nexus 4. I don't think Moto released the RAZR HD outside of Canada (and the CDMA Droid version in the US, of course).
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