1-First Reason: According to the Quran Allah has taken the responsibility to protect the message which was sent to Muhammad. This is one of the proofs that there is no Prophet/Messenger after Muhammad.
Here are the verses which show how Allah protects his message:
ALLAH ONLY SECURES HIS REVELATIONS
22:52 And We did not send before you any messenger or prophet, without having the satan interfere with his wishes. Allah then overrides what the devil has cast, and Allah secures His revelations. And Allah is Knower, Wise.
ALLAH IS GUARDIAN OF THE QURAN
15:9 Surely we have revealed the Reminder (The Quran) and we will most surely be its guardian.
NO CONTRADICTIONS IN THE QURAN
4:82 “Do they not ponder on the Quran? Had it been from other than Allah, they would surely have found in it many contradictions.”
NO FALSEHOOD COULD ENTER IT (THE QURAN), PRESENTLY OR AFTERWARDS
41:42 No falsehood could enter it, Presently or Afterwards; a revelation from One Most Wise, Praiseworthy.
2-Second Reason: According to the Quran Muhammad was sent to all people not just the Arabs. This is one of the proofs that there is no Prophet/Messenger after Muhammad.
Here are the verses where Allah said to Muhammad to tell the people that he has come as a Messenger to all people.
MUHAMMAD WAS SENT TO ALL PEOPLE
7:158 SAY: “O PEOPLE, I AM A MESSENGER OF ALLAH TO YOU ALL. The One who has the sovereignty of the heavens and the earth, there is no Allah except He; He gives life and causes death.” So believe in Allah and His Gentile messenger prophet, who believes in Allah and His words; and follow him that you may be guided.
WE HAVE SENT YOU TO ALL PEOPLE
34:28 And We have sent you (O Muhammad) to all people to be a bearer of good news, and as a warner; but most of the people do not know.
3-Third Reason: According to the Quran Muhammad was also sent to other people. These other people may include the people of later generations who will come after Muhammad. This is one of the proofs that there is no Prophet/Messenger after Muhammad.
MUHAMMAD WAS ALSO SENT TO OTHER PEOPLE
62:2 He is the One who sent to the Gentiles a messenger from among themselves, to recite to them His revelations, and to purify them, and teach them the Book and the wisdom. And before this, they were clearly astray.
62:3 And TO OTHERS FROM AMONG THEM WHO HAVE NOT YET JOINED THEM. He is the Noble, the Wise.
4-Fourth Reason: According to the Quran Allah said for each period there is a book. We know that Muhammad was sent to all people and to other people after him so this means that Quran is a book for the period begins from Prophet’s time till the end of time. This is one of the proofs that there is no Prophet/Messenger after Muhammad.
FOR EACH PERIOD THERE IS A BOOK
13:38 And We have sent messengers before you and We have made for them mates and offspring. It was not for a messenger to come with any sign except with the permission of Allah; FOR EACH PERIOD THERE IS A BOOK.