The main reason was Jerusalem is an isolated high point, you do NOT need to control it to take any of the three main roads though Palestine/Israel/Judea to get from Egypt to Damascus, but it is a good based to Attack/Control such roads from any enemy forces from Syria or Egypt. Thus as long a no substantial troops are stationed in Jerusalem it has little military significance, but given its location a difficult military objective only needed if you intend to rule Palestine/Israel/Judea NOT just travel through it to more profitable areas (as was Napoleon was doing).
There is a debate on Mecca at the time of Mohammad. Had the caravan route from Yemen to Mecca to Medina to Palestine/Israel/Judea died out or was it still functioning at that time. Ethiopia had invaded and then withdrawn as the Persian and Roman Empire was fighting their battle to their deaths (Persia would lose and be conquered by the Arabs, the Greeks would win, and be pushed out of Egypt, Palestine and Syria by the Arabs). No one seems to have used the Red Sea for sooner or later you had to disembark from the ships and hit the Caravan route anyway (For 1000 years before most trade via Egypt had gone on the "canal" built by the Persian around 500 BC just after the Persian conquered Egypt for the first time, rebuilt several times including just before the lost of Egypt by the Roman/Byzantines/Greeks).
Anyway this seems to have been the main business of Mecca for centuries, but the debate is how much the trade had declined given the Persian-Greek war of the early 600s. It was a massive war, ending in the complete defeat of Persia by the Greeks, but a Greek Empire no longer capable of any further offensives action, including even trying to hold Persia (Which the Greeks never tried) Egypt (Which the Greeks tried, but quickly gave up, through did invade to retake it the year after it fell, but then left again) but did try to hold onto Syria (But lost it after losing the cities to the Arabs, while the rural areas seemed to embrace the Arabs as soon as the arabs invaded, few conversions, but support.
My point is it is hard to compare cities to other cities, most exist for some geographical reasons that applies only to that city (Mecca is on a Orris on the Caravan route from Yemen to Jerusalem). Istanbul in on the trade route between the Black sea and the Mediterranean, Jerusalem is on the edge of the farming line in Palestine/Israel/Judea and thus where a division occurred. In Desert areas, the fear by attack by Bedouins, so the need for Caravans. In farming areas such Bedouins were not tolerated so the need for a caravan did not exist (You could go as an individual in the farming areas, of you were attacked and could hold off the attackers the local farmers would help you, thus most thieves were small groups and left as soon as possible. In the deserts you did not have any local farmers, so thieves could, and did, operate on much larger numbers and do more extensive attempts at robbery, thus people traveled in groups (Caravans) for mutual protection. Thus you have the Caravan route from Yemen to Mecca to Medina to Jerusalem or Amman. From Amman the caravan would go to Damascus. From Jerusalem or Damascus individual traders would take the merchandise through the farming belt to the coast for shipment elsewhere.
Thus it is hard to judge one city with another, all are different, often to different for comparison like you are making.
first spot to get access to farming items including improved pasture and water for whatever animals are being used to haul items across the Desert. Ancient Philadelphia (Modern Amman) was another Orrises in the Desert, a good spot to stop and take a break but like Mecca more stop over then a place to disperse (Which is what Jerusalem and Damascus were noted for.
http://en.wikipedia.org/wiki/Amman