My guess is that UFC wants to stay more on the side of Boxing as opposed to Wrestling. Although both sports are very successful, i think UFC sees itself as "youth's answer" to making boxing more exciting. So they steal the youth audience away from WWE and turn it into a highly-profitable, more corporate-style, event series that has more flair than boxing.
Therefore i think anything that resembles WWE (multiple people in the ring, tag-teams, etc.) will be avoided.