Because you haven't tought completely about it.
Cage favours the wrestlers much more. If you are beeing pushed to the fence, you cant defend from takedowns by sticking your butt back and going to half-squat because the fence gives your no space. Also you can't sprawl when you are near the fence. In a ring you can push your legs out under the ropes, and body towards the opponent and sprawl, ropes are elastic so you can defend from wrestling by pushing your butt back and squat.
Cage favours wrestler and heavier fighters. There is much more clinch and fence pushing in unified rules and longer time in that situations are tolerated by refs, so body mass / weight is much bigger advantage. Space of the cage is also much bigger and wider with no corners, so it is much more difficult for quality strikers to hit anyone and place their strikes because it is much easier to escape and move around and avoid strikes.
Cage favours fighters with big reach advanatage like Anderson or Jones who due to their reach and lot's of space to move can't be pressured or hit. They can maintain the distance very easy and strike from safe. It is almost impossible to hit fighters like Jones or Anderson in the octagon, you simply cannot reach them and cannot close them.
Elbows are bad for jiu-jits and ground game. Elbows and fence are reasons why quality ground game is almost estinguished in UFC. It is almost impossible to defend both elbows and fists so nobody wants to be on the bottom, in guard any more. Also fighters on top are not looking for submissions and gorund game because it is much easier to just ground and pound with elbows.
Also big parts of MMA like using knees and kicks and stomps on a grounded opponents do not exist in unified rules.
Fights in a ring are much more dynamic. There are more weapons fighters can use to win the fight. Wrestling is still strong, but quallity striking and BJJ are not in that big disadvantage and chances are more equal. That's why MMA in the ring is more interesting than in the cage.
