CO/NW, No codeshare in Micronesia
 

NW has NRT/GUM, NRT/SPN daily.
CO Micronesia does NRT/GUM 3 daily.

NW flies NRT/HNL 2 daily,KIX/HNL daily.
CO no longer flies NRT/HNL, but has GUM/HNL route.

CO has frequent GUM/SPN jet service.
NW airlink flies GUM/ROP/SPN frequently.

None of above code-Shared.
But Why?

It is unfortunate indeed for those of us who live in the Mariana Islands or travel here.

Perhaps it was a condition of federal governmental approval of the CO/NW partnership, in that those two carriers are the only U.S. flag airlines to serve these parts of the United States (Guam and the CNMI).

I don't know who has such regulatory approval. Who killed the UA/US merger? FTC? DoJ Antitrust Division? I've read press accounts that it is the latter reviewing the UA/US codeshare proposal. So if you find out who has regulatory oversight, and if such conditional approval documents are public, you could find your answer that way.

Alternatively, this could have been an agreement between CO/NW to ensure that neither carrier poaches the other's frequent flyers.

Finally, inasmuch as this part of the U.S. is often neglected by corporate headquarters -- we're neither fish nor fowl, "domestic" for some purposes and "international" for others -- it is quite possible that we've just been overlooked.

In the latter case, some suggestions to the carriers might have the desired effect.