"Whenever the Prophet...returned from a journey and observed the walls of Madinah, he would make his Mount go fast, and if he was on an animal (i.e., a horse), he would make it gallop because of his love for Madinah." (Sahih Bukhari, Book 30: Hadith 110)
The reason for the existence of most of the world's cities is obvious to geography students. New York and Shanghai control deep ports and straddle great rivers, bringing trade from the interior; Paris and London are at the crossing points of major cross-country rivers; Johannesburg sits atop a great mountain of gold ore; and Moscow and Madrid are at the heart of their great nations, easily able to control even the more distant corners of the land. Medina, however, is starkly different.
Even centuries before the birth of the Prophet Muhammad, Jews settled on the land to escape persecution at the hands of the Babylonians and Romans, meaning the area and its arid environment brought inhabitants precisely because it wasn't an attractive or resource rich area. In fact, the city fated to become the second holiest city in Islam earned that spot simply by straddling trade routes to the religious city of Mecca, which brought traders and pilgrims in large enough numbers to make it a trade center.
Of course, Mecca is now best-known for being Islam's holiest city, and it is revered as the birthplace of Muhammad and the site where Allah first revealed the Qu'ran to him. However, the prophet spent a great deal of time in Medina, especially when he and his supporters found themselves in conflict with the Meccans. Using Medina as a base, Muhammad eventually took Mecca, and both cities became integral parts of the Caliphate that followed. This meant that even as history brought geopolitical changes, Medina remained a city of religious significance for all Muslims, and even though its turbulent history is often overlooked, many people are still quite familiar with the city.