The big impediment to joining an alliance is the need to change systems to meet alliance standards. This costs a lot of money, and was the reason EI left Oneworld -- they faced spending several million Euros to accomodate JL joining OW and had no expectations of getting any JL traffic. Over 95% of Oneworld traffic on EI was connecting from/to AA and BA, so EI decided that bilaterals with AA and BA would give most of the benefit for EI at a much lower cost. MX might be in the same position (they connect mostly with AA and IB). I suspect they also cannot meet the OW qualification requirements, making the financial impact of joining prohibitive in the short run.