Personally I have been to Istanbul and the only main reasons I can think of Constantinople did not fall much earlier because of its strategic location at the edge of peninsular surrounded by natural three water/sea barriers and its extensive aqueduct systems.
Imagine the constant barrage of attacks from the Muslim Empires namely Umayyad, Abbasid and Ottoman over the course of around 700 years. This is due to prophecy that Constantinople will eventually fall to the Muslim army and only the best army General will prevail[1].
The attacks and sieges was started during by the Umayyad Empire in 674 and only in the year 1463 Sultan Fatih managed to finally subdue Constantinople for good. The only other time Constantinople was conquered happened during the Crusades'sacking in the year 1204. The Crusade invasion is only temporary and only lasted until 1261.
But the turning points of both of the successful sieges are very different. The Fatih's armies do not have direct access from the mainland and they do not have significant number of Christian Latin supporters in the Byzantine Empire and inside the fortress, unlike the Crusaders. The Ottoman basically have to resort to clever maneuvers like literally carrying the warships over hills to overcome the multiple chains barracades along the Golden Horn. The Ottoman was victorious probably due to conventional military strength and tactics but oblivious on the importance of aqueducts system supporting the lifeline of the city. The Crusades however, probably much aware of the importance aqueducts to the city due to their significant number of local Christian Latin supporters and managed to strangle Constantinople out of its lifeline. The recapture of Constantinople from the Crusade to reinstate the Byzantine Empire, however, is much easier because the Nicaean Empire obviously know the city inside out and the aqueduct systems.
This is my pure speculation based on my limited readings and observations of Istanbul, and more research needed to verify the potential causes of the fall of Constantinople and the importance of its aqueduct systems.
[1]https://www.dailysabah.com/feature/2015/05/29/the-conquest-o...