Salam to Nadia
"Just a question...How do we know the people of Mecca / whichever city the prophet was from did not ask God for forgiveness thus delaying/forgoing His retribution?"
There is no verse in the Quran, which says that the Meccans did ask for forgiveness of their sins and neither there is any verse, which says that they did not ask for forgiveness.
But if we refer to history we find that the Meccans never ceased to attack the Muslims till the pact of Hudaibiya was signed and this is mentioned in Surah 48 and this verse was revealed precisely in the year 628 AD.(six years after migration)
The Meccans later broke the peace of treaty in the attack which one of their allied tribes (the Banu Bakr) made on the Muslim Banu Khuza'a (who were in alliance with the prophet), but this led to the conquest of Mecca and the sweeping away of the autocracy.
It is clear that they never repented and neither prayed for their forgiveness but were bent upon destroying Islam until they were invaded, attacked, and subjugated.
And Verily, they were about to frighten you so much as to drive you out from the land. But in that case they would not have stayed (therein) after you, except for a little while. 17:76
This was not the first time that the pagans tried to drive out the prophet from Mecca but they tried many times and finally they succeeded in their mission
And (remember) when the disbelievers plotted against you (O Muhammad SAW) to imprison you, or to kill you, or to get you out (from your home, i.e. Makkah); they were plotting and Allah too was planning, and Allah is the Best of the planners. 8:30
The planning of the pagans to drive out the prophet or kill him or imprison him were round the clock and this forced the prophet to leave Mecca and lead to a safer place Madina as a refugee.
SOME MORE VERSES ON " why Mecca was not destroyed"
Say (O Muhammad SAW to the disbelievers): "My Lord pays attention to you only because of your invocation to Him. BUT NOW YOU HAVE INDEED DENIED (Him). So the TORMENT will be yours for ever (inseparable permanent punishment)." 25:77
Say (to the rejecters): "My Lord is not uneasy because of you if ye call not on Him BUT YE HAVE INDEED REJECTED (Him) and soon will come the INEVITABLE (punishment)!" 25:77
Say thou: my Lord careth not for you were it not for your prayer. Where as YE HAVE EVEN BELIED, so presently this denial shall come as the cleaving PUNISHMENT. 25:77
The promised PUNISHMENT never came upon the people of Mecca
And they ask you to hasten on the TORMENT (for them), and had it not been for a term appointed, the torment would CERTAINLY have come to them. And SURELY, it will COME UPON THEM suddenly while they perceive not! 29:53
They ask thee to hasten on the PUNISHMENT (for them): had it not been for a term (of respite) appointed the Punishment would CERTAINLY have come to them: and CERTAINLY it willREACH THEM of a sudden while they perceive not! 29:53
And they ask thee to hasten on the torment. And had not there been a term appointed, the TORMENT would SURELY have come unto them. And SURELY it SHALL COME UPON THEM of a sudden while they perceive not. 29:53
The SURE and CERTAIN PUNISHMENT is YET TO COME upon the PEOPLE of MECCA
All the verses I quoted goes against our belief that prophet was from Mecca. Had he been from Mecca then surely Mecca would have been destroyed.
And nothing prevents men from believing, now when the guidance (the Qur'an) has come to them, and from asking Forgiveness of their Lord, except that the ways of the ancients be repeated with them (i.e. their destruction decreed by Allah), or the torment be brought to them face to face? 18:55
The Meccans did not believe, did not pray for their forgiveness but continued their fight with the believers till they were invaded, attacked and conquered therefore the SUNNAH of Allah has to be repeated on the people of Mecca and should be punished for their disbelief and hypocrisy.