Why are there are only one Buddha per universe?

Because of focus. It is easier to rely on one buddha 24 hours a day 7 days a week.

please don't consider Buddha as person or reincarnation of any sort of external power. every one has potential to become Buddha.become a Buddha is completely depend on commitment.its a states of mind any one can achieved by own human effort and commitment. danuka form sri lanka

The final truth is that there is only one buddha... that can emanate as millions of buddhas. Think of how the buddha Green Tara emanated from the tears of Avalokiteshvara. And then 21 Taras emanate from Tara. Etc.

According to Theravada tradition, Buddhas come and go over countless eons. Only in the times when their teaching is alive and true will we get a chance of escaping from this endless samsara. Every once in a great while, after a long period of spiritual darkness blankets the world, an individual is eventually born who, through his own efforts, rediscovers the long-forgotten path to Awakening and liberates himself once and for all from the long round of rebirth, thereby becoming an arahant ("worthy one," one who has fully realized Awakening).

In Digha Nikáya Sutta 14, Mahapadana Sutta (The Great Discourse on the Lineage)) & DN 32 The Ātaānātiiya Discourse, the Supreme Buddha stated that six Supreme Buddhas appeared over 91 world-cycles. The seven Buddhas, including "our" Supreme Buddha, mentioned in DN 14 & DN 32: Vipassi, Sikhi, Vessabhu, Kakusandha, Konagamana, Kassapa, and Gotama. They were all born in this earth, in the land of the Rose Apple (Jambudipa), in north-central India in the area known then as the Middle Land (Majjhima Desa).

As per the scriptures the next Buddha to come will only come to pass when the life expectancy of mankind is 80,000 years. To come to the question proper... the very question that you asked was once asked by King Milinda, and Arahant Nagasena replied to it as posted in the earlier answer by Dhammadhatu.

Also if you would want to read Bahudhātuka sutta (MN. 115), Mahāgovinda sutta (DN. 19), Sampasādanīya sutta (DN. 28), etc. Buddha has said:

"He [i.e., the bhikkhu who understands the possible and the impossible] understands: ‘It is impossible, it cannot happen that two Arahants who are Sammā-sambuddhas could arise contemporaneously in one world-system — there is no such possibility.’"

Buddha fully understood the universe when He declared this, and this is what is said in the scriptures:

According to the Scriptures the universe is infinite in time and space. By infinite what is meant is the length, width or number of solar systems (lokadhātu) in the universe are infinite. Within that infinite space there are sections that consist of one billion solar systems are called “Thri Sahashri Loka Datu” (equivalent to a galaxy). Such unfathomable spaces are always explained by similes. One such is - If someone takes mustard seeds amount equal to the volume of Thri Sahashri Loka Datu (equivalent to a galaxy) and travel in one direction placing one seed on one solar system, the seeds will come to an end but not the universe.

Buddha has not refereed to space as impermanent or not, but all things within it are. All the worlds of the devas, and the Brahma worlds are found within this time and space. For instance the scriptures talk about realms of the gods of the base consisting of the infinity of space... realms of the gods of the base consisting of the infinity of consciousness... realms of the gods of the base consisting of nothingness...

All of us beings of all these diverse worlds, whatever we do in time and space is going to come to pass sooner or later, like a picture you draw with a stick in flowing water. But there is one realm outside of time and space, Nibbhana, that Buddha wants us to reach, where we find the deathless. Once you’ve found the Deathless it’s always there as it is beyond time and space.

A collection of 1000 solar systems are called “Sahashri Loka Dhatu” in the scriptures. A collection of 1,000,000 solar systems are called “Divi Sahashri Loka Dhatu”. The universe may not be impermanent (as there is no mention of it). Within it, the solar systems are born, gets destroyed and it will be made again. Such cycles are called “Maha Kalpa(great kalpa)”.

Maha Kalpa are further divided: time of solar system formation ( Sanwatta Kalpa), time of existence (Sanwatta Sthayi Kalpa), time of destruction (Wiwatta kalpa) and time of emptiness (Wiwatta Sthayi Kalpa). The time intervals are termed “Asankya Kalpa”.

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

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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