"Big" John McCarthy explains:

First, if a titleholder from the UFC fights either in the UFC or in another promotion and loses to a fighter not under contract to the UFC, then the UFC just devalued itself and its championship belt. They will not have control of the person who is now considered the top fighter in the world and their belt will be viewed by many people as just another promotional belt instead of the belt that signifies the best fighter in the world at that weight class. People will realize that maybe not all of the greatest fighters in the world fight for the UFC, but that there are other promotions out there that have very good fighters who can fight as good or better than the more popular fighters under contract to Zuffa.

None of this helps the UFC so you can see why they will not be jumping on the WAMMA bandwagon. I can't blame Zuffa at all for looking at this the way it does. They have worked very hard to get where they are. They have their matchmaker in Joe Silva who sets up the matches that Dana White, the president of the UFC, wants to promote. They are a very successful self-contained promotion that has no outside influences telling them what to do. Why would they want to let someone in that can now influence their promotion in anyway?

This is the problem that confronts Dave Szade and Michael Lynch, the chief executive officer and chief operating officer, respectively, of WAMMA. They have a pretty good idea, and I want to see the best fighters in the world fighting each other no matter what promotion they are signed to. Politics and money have a huge influence over all of the combative sports and since that is not going to change, it is going to take a "hell freezing over" moment or a shift in the political power of the sport before you see the UFC and WAMMA working together.

McCarthy is actually describing why the UFC won't join WAMMA, but the truth is there need to be rivals to the UFC or at least moderate competitors of a lesser scale who are willing to work together for WAMMA to gain any traction. That, it seems, is not so easy to pull off. Entities that have tried to go head to head with the UFC are new, suffering or dead (BodogFIGHT, IFL, the Pro Elite federation; too early to tell with Affliction). But the other point of note is that if any rival to the UFC was able to gain some momentum, their incentives to join and work with WAMMA would be begin to resemble the UFCs as aforementioned. WAMMA won't admit that this is their aim, but they are trying to realign the power positioning of the MMA landscape for rival organizations in a quest for legitimacy. They will reply that WAMMA wants to work with the UFC, but WAMMA aim's are antithetical to much of what the UFC is trying to accomplish. The UFC knows they don't have a monopoly on talent and the MMA fans and general public are unaware of many talented competitors in other promotions. To legitimize WAMMA is to legitimize the UFC's competition in a way co-promotion could only hope to do.