Here's why:
* Master of Pendulum's effect states: "While this card is face-up on the field, neither player can Special Summon monsters."
* Monster Reborn's effect lets you Special Summon a monster from your graveyard.
Since Master of Pendulum prevents Special Summoning, you can't use Monster Reborn to bring back Blue-Eyes Ultimate Dragon. This is true regardless of the card's original summon method (Normal Summon, Fusion Summon, etc.).
Important Note: Monster Reborn would be able to bring back Blue-Eyes Ultimate Dragon if Master of Pendulum was not on the field.