Well the professional career of a MMA fighter is so short that these fighters want to make as much money as possible in that short career span. The PPV cut that Alvarez has in his UFC contract might get as high as 7~10 times of his base pay. Also, joining the UFC would mean much more exposure and that would mean more big money sponsors. Alvarez would make much more money in the UFC.
I agree that he has the "potential" to make more than the UFC but it isn't 100%
If he loses then he isn't going to make more in the UFC. LW division is extremely deep and Eddie gets dropped in every single fight he is in.
The bidding/matching war has driven up his value in Belletor where he now has a VERY good contract. He can make VERY good money fighting much easier competition. Sure, his earning potential is higher in the UFC, but that is dependent on actually winning against MUCH better fighters.
From a strictly monetary standpoint he could very well be better off being a big fish in a small pond, rather than a middle of the road fighter in the UFC.
I am not convinced he would be champion in the UFC, and I think there are plenty of guys that would beat Eddie fairly easily in the division.
Same reason why Khalidov is still fighting in Poland instead of the UFC.