Why are there are only one Buddha per universe? Why there are only one Buddha per universe?

And why there are not more or less at any time?
The teaching attributed to the Buddha of 'there can only be one self-enlightened Buddha in a world-system (lokadhātuyā)' is found in the Bahudhātuka Sutta of the Majjhima Nikaya, which states:


  He understands: ‘It is impossible, it cannot happen that two Accomplished Ones (arahanto), Fully Self-Enlightened Ones (sammāsambuddhā), could arise
  contemporaneously in one (ekissā) world-system (lokadhātuyā) ― there is no such possibility.’
  
  And he understands: ‘It is possible that one Accomplished One, a Fully
  Self-Enlightened One, might arise in one world-system ― there is such a
  possibility.’
  
  Bahudhātuka Sutta
  
  




The Milindapañha, which is a later-day commentary (c. 100BC), provides a detailed explanation: 


  This world system, O king, is a one-Buddha-supporting world; that is, it can bear the virtue of only a single Tathāgata. If a second
  Tathāgata were to arise the world could not bear him, it would shake
  and tremble, it would bend, this way and that, it would disperse,
  scatter into pieces, dissolve, be utterly destroyed. just as a boat, O
  king, might be able to carry one passenger across. Then, when one man
  had got on board, it would be well trimmed and able to bear his
  weight. But if a second man were to come like to the first in age and
  caste and strength and size and stoutness of body and build of frame,
  and he too should get on board the boat—would that boat be able, O
  king, to carry them both?
  
  Or suppose, O king, that a man had eaten as much food as he wanted, even so that he had filled himself with nourishment up to the
  throat, and he—thus satiated, regaled, filled with good cheer, with no
  room left for more, drowsy and stiff as a stick one cannot bend—were
  again to eat as much food as he had eaten before—would such a man, O
  king, then be at ease?’
  
  Suppose, O king, there were two carts quite filled with precious things up to the top, and people were to take the things from the one
  cart and pile them up on the other, would that one be able to carry
  the weight of both?’
  
  If, O king, two Buddhas were to arise together, then would disputes
  arise between their followers, and at the words: “Your Buddha, our
  Buddha,” they would divide off into two parties—just as would the
  followers of two rival powerful ministers of state. This is the other reason, O king, why two Buddhas could not appear at the same
  time.
  
  The passage (of Scripture) that the Buddha is the chief would become false, and the passage that the Buddha takes precedence of all would
  become false, and the passage that the Buddha is the best of all would
  become false. And so all those passages where the Buddha is said to be
  the most excellent, the most exalted, the highest of all, the peerless
  one, without an equal, the matchless one, who hath neither counterpart
  nor rival—all would be proved false. Accept this reason too as in
  truth a reason why two Buddhas cannot arise at once.
  
  Of other things also, whatever is mighty in the world is singular. The broad earth is great, O king, and it is only one. The ocean is
  mighty, and it is only one. Sineru, the king of the mountains, is
  great; and it is only one. Space is mighty, and it is only one. Sakka
  (the king of the gods) is great, and he is only one. Mara (the Evil
  One, Death) is great, and he is only one. Mahā-Brahmā is mighty, and
  he is only one. 51 A Tathāgata, an Arahat Buddha supreme, is great,
  and he is alone in the world. Wherever any one of these spring up,
  then there is no room for a second. And therefore, O king, is it that
  only one Tathāgata, an Arahat Buddha supreme, can appear at one time
  in the world.’
  
  Mil 5.5 11; Mil 235