Why do you think Christianity spread more rapidly in the cities than in the countryside?

They did so for very different reasons.

The case of Islam is relatively straightforward; there is no rural countryside in Arabia, since Arabia is a desert. Towns and cities are where most people lived because that's where the water was. As a result all religions that developed in Arabia developed in towns and cities as opposed to the non-existent countryside.

Christianity developed as a faith through the efforts to evangelists. Unlike most polytheistic faiths, Christianity did not start as a nomadic tribe's local gods and goddesses that were expanded over larger distances. Christianity was created and developed by Christian leaders in a number of small councils and bishoprics. These councils would be formed in the Greek and Anatolian cities where Christianity was beginning to attract followers due to preaching there. Since the initial proselytization and the earliest churches and bishoprics were in cities, it makes sense that the initial successes of the Christian Church were in cities, rather than the countryside.