Constantinople was the city at the center of the Eastern Roman Empire (called by modern Historians the Byzantine Empire, but the Empire itself always referred to itself as the Roman Republic), but the source of most Military Recruits for the Eastern Empire, since the reforms of Heraclis (610-641 AD), had been Central Asia Minor (Antonia). With Manzikert that area the those recruits came more and more under Turkish control.
The Themes, generally thought to have been implemented by Heraclis, replaces the Roman Mercenary Cohort Legion, with a trained militia, whose service was tied in with land ownership (and the son of the Soldier had to serve in the Army to inherit the land, thus land ownership and military duty were directly connected as it had been under the Early Roman Republic before the "Reforms of Marius" in 109 BC that set up the Classic Roman Mercenary Cohort Legion).
The Themes were to stay the basic system of military recruitment in right is now Turkey till almost World War I (along with the even older Tax system for the area first implemented by Emperor Diocletian about 300 AD).
Thus the Battle of Mazikert was a disaster for the Byzantium Empire, they lost they main source of Recruits and lost them to the Turks. Within ten years all of Central Asia Minor was in the control of the Seljuk Turks and within 20 years the Eastern Emperor was asking the West for Assistance against the Turks. It was this request for Assistance that lead to the Call for the first Crusade, something the Byzantium Empire really did NOT want, they wanted assistance in Asia Minor NOT Palestine. But as Napoleon observed, the best enemy is a Coalition, for each member of the Coalition has its own agenda. Thus the Crusaders saw no advantage in helping Constantinople re-take parts of Asia Minor (Except as a way to get to Palestine). On the other hand The Crusaders did see an advantage in taking Palestine (Beside the Religious one) which was they could set up independent Kingdoms in Palestine, they were away from Constantinople.
Furthermore the Turks, while Moslem and concerned about who ruled Jerusalem, were more concerned about the area they ruled, Baghdad to Asia Minor then Palestine (Technically Palestine was Ruled by Egypt at the time, but it was a weak Shiite Government (The Fatimid Dynasty) that would NOT survive the Crusades for long. Except for a raid in 1169, no Crusaders ever attacked Egypt till the Fifth Crusades, over 200 years AFTER the first Crusade. Furthermore the 1169 Raid saw the Crusaders take and hold one small city on the Egyptian coast but lost that city afterward when Saladin attacked Palestine.
My point is I hate when someone points to an Historical event and does NOT put it in context. The Call for the First Crusade was the result of what had been happening over the previous 30 years in Asia Minor, something that is ignored in most comments on the Crusades.
More On Manziket
http://en.wikipedia.org/wiki/Battle_of_ManzikertThe battle of Myriokenphalon which, had the Byzantium won would have undid most of the harm of Manzikert, but the Greeks lost the battle and 30 years later their Capital to the Fourth Crusade (and with that lost, the Greeks ceased to be anything more then a minor impediment to the Ottoman Turks):
http://en.wikipedia.org/wiki/Battle_of_MyriokephalonOn Heraclis (Roman Emperor 610-641)
http://www.roman-emperors.org/heraclis.htmhttp://en.wikipedia.org/wiki/HeracliusThe Byzantium Themes:
http://en.wikipedia.org/wiki/Theme_%28Byzantine_administrative_unit%29Fifth Crusade:
http://en.wikipedia.org/wiki/Fifth_CrusadeThe Fatimid Dynasty of Egypt:
http://en.wikipedia.org/wiki/Fatimid