Sheikh Bakr Zakî `Awad
The reason that calling to Allah was more successful in Madinah is because the people of Madinah knew of the Messenger’s call from the Jews who were residing there. The Jews used to tell the people of Madînah that a Prophet will one day be sent. That is why, when he came to them, they were eager to believe in him.
The people of Mecca were totally submerged in paganism and this prevented their acceptance of the new call. They had inherited from their forfathers ideas that prejudiced them against the new religion while this was not the case for the people of Madinah.
We should recall that many people from Madinah accepted Islam while Prophet Muhammad (peace be upon him) was still in Mecca. Some of them accepted Islam when they came to Mecca for pilgrimage. They, in turn, invited others in Madinah to accept Islam upon their return. They are the ones who invited Prophet Muhammad (peace be upon him) to come to them in Madinah in the first place. However, guidance is only given by Allah, and he bestows it upon whomever He wills.
And Allah knows best.
Source: Islam Today