Good question. i will answer your question. i will be happy if it gives you satisfaction. Now we will go to the answer.
my answer:
your questions is this: Why couldn't be there no more Buddhas after Gotama Buddha? Good question.
The Avatar is necessary when a special work is to be done and in crises of the evolution.
An avatar is one who changes the course of human evolution; such as Buddha, Krishna, and Jesus.
Role of the Avatar
The avatar comes to bring new spiritual insights to the world or region that was not there before in order to bring progress for the society. Jesus taught a new level of universal and divine Love that was not there before, Buddha for the first time addressed the issue of suffering, death, and impermanence; and Krishna presented a new the nature of the Soul and how it can be a conduit to the Divine Reality. Adishankara came to uplift the people. Sri Aurobindo and the Mother came recently to usher in the new age of human progress through Conscious Evolution, culminating in a Divine Life on earth.
Rama avatar to show dharma to the people. Krishna avatar to establish dharma in the world. similarly, Bhuddha avatar to awake the people who were in suffering of sick, death etc., The avatars are incarnation of god. The god will come in avatara for the benefit of people. so, Bhuddha was an avatar came to earth to teach morality, moral principles, and philosophy.
This is just for example only. we born in this earth. we have one mother and one father. To whom we born as a child. we grow. we lost our father in our life due to age. so, there will be one father, one mother only to us. can we choose any other person as our father ? we can treat any person like our father, but not actually a father.
Avatars are known to be messiahs who come down from heaven to deliver a message of God to men. They are charged with a mission by the Godhead. Buddha, Krishna, Rama, and Jesus are called avatars. The avatar comes to bring new spiritual insights to the world or region that was not there before in order to bring progress for the society. Jesus taught a new level of universal and divine Love that had not existed before. Buddha for the first time addressed the issue of suffering, death, and impermanence head on, and devised a way out. Krishna presented anew the nature of the Soul and how it can be a conduit to the experience of the Divine Reality.
why no more Bhuddhas ?
At the age of 29, the Buddha stopped praying to the gods to end his suffering and the suffering of others. He left his family and friends, went to the edge of the forest, took off all his clothes and jewelry, covered his naked body with rags of cloth, cut off his hair and started to meditate.
He became a mendicant, and It took him six years of hard work and much suffering, but in the end he was able to stop his suffering forever (Nirvana) and help others stop their suffering as well. The Buddha was more concerned with the human condition: Birth, Sickness, Old age, and Death. The Buddhist path is about coming to a place of acceptance with these painful aspects of life, and not suffering through them. Because of his Nirvana, the Buddha was perfectly moral, perfectly ethical, and ended his suffering forever.
Men may come and men may go. All men in this world are not shining. so, an Avatar came, done the job perfectly, and left. it is our duty to follow. Avatar always one, and no more the same avatar.
so, Bhuddha came to earth, and fulfilled his aim. There will be only Bhuddha like one jesus, krishna, rama, and they came to earth, they done the work and gone. we simply can follow their goal. There is no need for us another Bhuddha since he left with a treasure of good teachings.
Thanks for the opportunity !!!