Practice of meditation seems to attract mostly inhabitants of big cities. Surely there are reasons for that, but we won't be discussing them now.
Sooner or later a practitioner of the Dhamma starts looking for like-minded people - to learn new things, to discuss their experiences and to feel support of the group. Sometimes those groups meet in someone's house or even in a flat, and in some cases a group may acquire a property for communal use. Hampstead Vihara in London was a good example of such property, where people could meet, meditate, and where eventually a group of monks headed by Ajahn Sumedho, was invited to take up residence.
Because it is located in a city, easily accessible to the townsfolk, a Dhamma Centre maintains it's "liveliness", being available for many kinds of activities: dropping in for a quiet break or for an evening meditation, meeting friends or picking up a book, attending a lecture or a day-long meditation course.
Forest monks and nuns would quickly find these conditions too busy and restrictive and at the first opportunity they will try to move to a peaceful, solitary place in nature. A Forest Monastery or a quiet retreat centre will be their preferred dwelling and a place to deepen their practice.
Gradually, the connection between inhabitants of a Forest Monastery and people in a big city weakens, it becomes more difficult for them to find time to travel to the countryside, and importance of a City Dhamma Centre becomes obvious. For example, Forest Monasteries of Ajahn Chah and Ajahn Buddhadasa have their Centres in Bangkok, and particularly "Buddhadasa Archives" became very important and influential place.