Abbas - Tanwîr al-Miqbâs min Tafsîr Ibn ‘Abbâs
(Those who disbelieve) in Muhammad (pbuh) and in the Qur'an (say: If only some portent sign were sent down upon him) why is not some sign sent to him (from his Lord!) to prove his prophethood as it was sent to earlier messengers (Thou art) O Muhammad (a warner only) a warning messenger, (and for every folk a guide) a prophet; it is also said that this means: a caller who calls them to leave error and follow guidance.
And those who disbelieve say, ‘Why has not some sign been sent down upon him, upon Muhammad (s), from his Lord!’, the like of the staff and the [glowing] hand, or the she-camel. God, exalted be He, says: You are only a warner, one to threaten the disbelievers, for it is not your duty to bring forth signs; and for every folk there is a guide, a prophet, to guide them to their Lord with the signs that He gives him, and not what with they request.
And those who disbelieve say, 'Why has not some sign been sent down upon him from his Lord!': having become veiled, they did not see the signs that bore witness to [Muḥammad's] prophethood, such as his [the Prophet's] assumption of God's attributes, because of their non-perception and the blindness of their insights. That is why they did not reckon these as signs and requested them according to their vain desires. Your duty is only to warn them, and not to guide them, for guidance belongs to God; and for every folk there is a guide, who corresponds to them in terms of the genus-correspondence of primordial nature and so they acquire an affinity with him at the point of his perfection and his reception of the divine light cast upon him. They accept guidance from him and so God guides them according to His manifestation [in that guide]. Therefore, whoever corresponds to you in terms of that original genus-correspondence will receive guidance from you, and whoever does not, will not.
Allah states that out of their disbelief and stubbornness, the idolators asked why is not a miracle sent down to the Messenger from his Lord, just like the earlier Messengers For instance, the disbelievers were being stubborn when they asked the Prophet to turn As-Safa into gold, to remove the mountains from around them, and to replace them with green fields and rivers. Allah said,
Maududi - Sayyid Abul Ala Maududi - Tafhim al-Qur'an
This showed their mentality. The only criterion of judging whether Muhammad (Allah's peace be upon him) was a true Prophet or not according to these disbelievers, was whether he would perform a miracle to order. This was because they would not judge his Message by rational arguments: they would not learn a lesson from his high character nor from the moral revolution that was being brought about among his Companions by his Message and noble example: nor would they carefully consider the rational arguments given in the Qur'an to prove the errors of their religion of shirk and superstitions of ignorance. They would ignore these rational things and demand a miracle to decide the issue.
This is the concise answer to their demand, though it has not been directly addressed to the disbelievers but to the Messenger. It is this: "O Prophet, you should not worry at all about some miracle which you might show to these people in order to convince them, for it is not a part of your Mission. Your duty is only to warn the people of their negligence and of the evil consequences of their wrong ways. And for this purpose, We have always sent a guide to every people. Now you, too, are performing this duty, and it is for them to open their eyes and judge the truth of your Message." After giving this concise answer, Allah overlooks their demand but warns them, in the succeeding verses, that they have to deal with the All-Knowing Allah Who has full knowledge about everyone of them from the time they were in the wombs of their mothers, and Who keeps a keen watch over everything they do. Therefore their fates shall be decided strictly with justice in accordance with their merits and that there is no power on the earth and in the heavens that can influence in the least His decisions and judgments.