The unbelieving Quraish in their campaign to spread lies and slander against the Holy Prophet were facing a real difficulty. They did not know how to account for the wonderful discourses which were being presented before the people in the form of the Qur'an and which were moving their hearts deeply. They could not stop the Qur'an from reaching the people. The only thing they could do to counteract its effect and influence was to create doubts and suspicions about it in their minds and hearts. Therefore, in their desperation they charged that Muhammad (Allah's peace be upon him) was a sorcerer, who was being inspired by the satans, and they considered this charge of theirs to be the most effective because it could neither be easily verified nor refuted.
Tafsir Ibn-Kathir: (The Qur'an was brought down by Jibril, not Shaytan) Allah tells us about His Book, which falsehood cannot approach from before or behind it, sent down by the All-Wise, Worthy of all praise. He states that it has been brought down by the trustworthy Ruh (i.e., Jibril) who is helped by Allah.
(And it is not the Shayatin who have brought it down.) Then He tells us that it could not be the case for three reasons that the Shayatin brought it down. One is that it would not suit them, i.e., they have no desire to do so and they do not want to, because their nature is to corrupt and misguide people, but this contains words enjoining what is right and forbidding what is evil, and light, guidance and mighty proofs.
There is a big difference between this and the Shayatin, Allah says: (Neither would it suit them) (nor are they able.) meaning, even if they wanted to, they could not do it.
Allah says: (Had We sent down this Qur'an on a mountain, you would surely have seen it humbling itself and rent asunder by the fear of Allah) (59:21). Then Allah explains that even if they wanted to and were able to bear it and convey it, they still would not be able to achieve that, because they were prevented from hearing the Qur'an when it was brought down, for the heavens were filled with guardians and shooting stars at the time when the Qur'an was being revealed to the Messenger of Allah , so none of the Shayatin could hear even one letter of it, lest there be any confusion in the matter. This is a part of Allah's mercy towards His servants, protection of His Laws, and support for His Book and His Messenger .
Yusuf Ali Explanation:
When anything extraordinary happens, there are always people desirous of putting the worst construction on it, and saying that it is the work of Satans. So when the Qur'an came with its Message in wondrous Arabic, its enemies could only account for its power by attributing it to evil spirits! Such a beneficent message can never suit the purposes of Satans, nor would it be in their power to produce it. In fact Good and Evil are poles asunder, and Evil cannot even hear words of Good, of tender Pity for sinners and Forgiveness for the penitent!
وَمَا يَنۡۢبَغِىۡ لَهُمۡ وَمَا يَسۡتَطِيۡعُوۡنَؕ
(26:211) nor does it behove them, nor does it lie in their power.
That is, these revelations and themes do not at all suit the satans. Any person who has a little common sense can well understand that the sublime discourses being presented in the Qur'an cannot be inspired by the satans. Never has it happened that the satans might have taught the people through the sorcerers to worship God and fear Him, or forbidden them from shirk and idol-worship, or warned them of the accountability of the Hereafter, or prohibited them from tyranny and sexual and moral evils, and exhorted them instead to act righteously and do good to others. Such works cannot behoove the satans. Their only pastime can be to sow the seeds of discord among the people and to arouse them to mischief and vice. The common observation is that people visit the sorcerers to find out whether they will succeed in their love affairs or not, what move would suit them in gambling, what trick and stratagem would be helpful against the enemy, or who had stolen the camel of so and so. Apart from such affairs and problems, the sorcerers and their patron-saints cannot be expected to worry themselves about matters like reforming the people, teaching them morals and cleansing their lives of vice and evil.
Even if the satans wanted they could not impart Truth and Goodness to the people like a true teacher and reformer as the Qur'an does. Even if to deceive the people, they came out under the guise of a benefactor, their work would not be tree from blemishes, which would betray their ignorance and their hidden satanic nature. Similarly the life and the teachings of a person who poses to be a religious guide, under the influence and inspiration of satans, would inevitably reflect the wickedness of intention and design. Thus the satans can neither inspire others with piety and goodness, nor can those, who have any relation with the satans, become pious and righteous themselves. Then in addition to its high and noble teachings, the Qur'an is a lucid and eloquent Book, which contains the knowledge of Reality. That is why it has over and over again put forward the challenge that human beings and jinns would never be able to produce a book like the Qur'an even if they collaborated with all their energies and capacities. "Declare this: Even if human beings and jinns should cooperate with one another to bring forth a book like the Qur'an, they will never be able to bring anything like it, even though all of them help one another" (Bani Isra'il: 88).