While I think UA's relationship with Chase is an obstacle today there must be more to it. To the best of my recollection UA has never participated in Membership Rewards even going back to the predecessor program called Membership Miles. It may simply be that the (then) first tier domestic carriers, AA and UA, had much less to gain from this type of program than the smaller domestic carriers and foreign carriers.
Mia, you may be correct. I just wish that at least for the elite levels of AMEX, that there would be some cooperation between the programs. To me, and I believe, many others, that would be a true "enhancement" to the program. You have the worlds second largest airline, and the largest rewards program, it seems to me that some sort of cooperation would really benefit both.
I wish that I new the full story and the histories of the two programs. The Chase angle is the only thing I have heard and it makes sense.