Another reason why people don’t believe in the message that the prophet Muhammad (or any other messenger for that matter) was sent with, is that he was a man.
And We sent among them a messenger from themselves, [saying], “Worship Allah; you have no deity other than Him; then will you not fear Him?”
And the eminent among his people who disbelieved and denied the meeting of the Hereafter while We had given them luxury in the worldly life said, “This is not but a man like yourselves. He eats of that from which you eat and drinks of what you drink.
And if you should obey a man like yourselves, indeed, you would then be losers.
Does he promise you that when you have died and become dust and bones that you will be brought forth [once more]?
How far, how far, is that which you are promised.
Life is not but our worldly life – we die and live, but we will not be resurrected.
He is not but a man who has invented a lie about Allah, and we will not believe him.”
Surah Al-Muminun (23 – 32-38)
Has there not come to you the news of those who disbelieved before? So they tasted the bad consequence of their affair, and they will have a painful punishment.
That is because their messengers used to come to them with clear evidences, but they said, “Shall human beings guide us?” and disbelieved and turned away. And Allah dispensed [with them]; and Allah is Free of need and Praiseworthy.
Surah At-Taghabun (64 – 5-6)
And We have certainly diversified for the people in this Qur’an from every [kind] of example, but most of the people refused [anything] except disbelief.
And they say, “We will not believe you until you break open for us from the ground a spring.
Or [until] you have a garden of palm trees and grapes and make rivers gush forth within them in force [and abundance]
Or you make the heaven fall upon us in fragments as you have claimed or you bring Allah and the angels before [us]
Or you have a house of gold or you ascend into the sky. And [even then], we will not believe in your ascension until you bring down to us a book we may read.” Say, “Exalted is my Lord! Was I ever but a human messenger?”
And what prevented the people from believing when guidance came to them except that they said, “Has Allah sent a human messenger?”
Say, “If there were upon the earth angels walking securely, We would have sent down to them from the heaven an angel [as a] messenger.”
Say, “Sufficient is Allah as Witness between me and you. Indeed he is ever, concerning His servants, Acquainted and Seeing.”
Surah Al-Isra (17 – 89-96)
If a message is so important, it should have come down another more prestigious way, not in text…
And thus We have placed within every city the greatest of its criminals to conspire therein. But they conspire not except against themselves, and they perceive [it] not.
And when a sign comes to them, they say, “Never will we believe until we are given like that which was given to the messengers of Allah .” Allah is most knowing of where He places His message. There will afflict those who committed crimes debasement before Allah and severe punishment for what they used to conspire.
Surah Al-Anam (6 – 123-124)
And they say, “O you upon whom the message has been sent down, indeed you are mad.
Why do you not bring us the angels, if you should be among the truthful?”
We do not send down the angels except with truth; and the disbelievers would not then be reprieved.
Indeed, it is We who sent down the Qur’an and indeed, We will be its guardian.
And We had certainly sent [messengers] before you, [O Muhammad], among the sects of the former peoples.
And no messenger would come to them except that they ridiculed him.
Thus do We insert denial into the hearts of the criminals.
They will not believe in it, while there has already occurred the precedent of the former peoples.
And [even] if We opened to them a gate from the heaven and they continued therein to ascend,
They would still have said: “Surely our eyes have been dazzled; rather, we have been enchanted.”
Surah Al-Hijr (15 – 6-15)
And those who disbelieve say, “Why was the Qur’an not revealed to him all at once?” Thus [it is] that We may strengthen thereby your heart. And We have spaced it distinctly.
Surah Al-Furquan (25 – 32)
However, some use this to their convenience by accusing the prophet Muhammad of conjuring up the Quran as he goes along, as he sees fit. If this was the case, he would not have passed five long months without receiving revelation. When his own tribe was saying that Allah has abandoned him; he should have just invented anything, but he couldn’t, wouldn’t, and didn’t.
[That] indeed, the Qur’an is the word of a noble Messenger.
And it is not the word of a poet; little do you believe.
Nor the word of a soothsayer; little do you remember.
[It is] a revelation from the Lord of the worlds.
And if Muhammad had made up about Us some [false] sayings,
We would have seized him by the right hand;
Then We would have cut from him the aorta.
And there is no one of you who could prevent [Us] from him.
Surah Al-Haqqah (69 – 40-47)
There are multiple reasons for not having revelation brought down in one piece. It allowed people to assimilate it (memorize and practice what they preach). Some laws (such as outlawing alcohol) required a gradation. And when people had questions concerning faith or day to day life; revelation on that subject made it seem personal in its nature just as it is universal in its reach.
Or the message itself should be grander than just a book…
Rather, the Qur’an is distinct verses [preserved] within the breasts of those who have been given knowledge. And none reject Our verses except the wrongdoers.
But they say, “Why are not signs sent down to him from his Lord?” Say, “The signs are only with Allah, and I am only a clear warner.”
And is it not sufficient for them that We revealed to you the Book which is recited to them? Indeed in that is a mercy and reminder for a people who believe.
Surah Al-Ankabut (29 – 49-51)
Rather, every person among them desires that he would be given scriptures spread about.
Surah Al-Muddatthir (74 – 52)
And they swear by Allah their strongest oaths that if a sign came to them, they would surely believe in it. Say, “The signs are only with Allah .” And what will make you perceive that even if a sign came, they would not believe.
And We will turn away their hearts and their eyes just as they refused to believe in it the first time. And We will leave them in their transgression, wandering blindly.
And even if We had sent down to them the angels [with the message] and the dead spoke to them [of it] and We gathered together every [created] thing in front of them, they would not believe unless Allah should will. But most of them, [of that], are ignorant.
Surah Al-Anam (6 – 109-111)