It may have had some of the most impressive fortifications, but the small garrison meant that they would not be sufficiently manned. The sieges beforehand were not always failed conquests but application of political pressure to get the byz to be loyal tribute payers, I haven't checked recently but IIRC not all of the previous sieges had the dardanelles blocked off like Mehmed was able to
Ehh Malta, Belgrade, Rhodes, and Corfu all held off comparable Ottoman armies for longer with smaller garrisons and less impressive fortifications. And the defenders force multipliers are not to be underestimated. At Koseg you had under a thousand troops, with no cannons, repulsing 19 separate assaults over four weeks from an Ottoman army of around 100,000; after which the Turks withdrew because keeping your army out in enemy the border lands in front a fort in denuded country was a great way to lose it.
In 1391 and 1394 the Ottomans had tried to blockade at sea but the Christians rallied sufficient fleets to overcome the blockades. In 1411 and 1422 the Ottomans were less well prepared for naval blockade. 1453 was certainly the best match up of technology, manpower, and seapower that the Ottomans had ever had. But would have only taken one bad plague outbreak to break the siege. Or an invasion by the Hungarians. Or a significant rebellion. Mehmed II was not an idiot, he ordered a frontal assault for a reason rather than waiting for further attrition among the garrison or further breakthroughs with the siege guns. Even then, all accounts are that the assault was a close run thing (with multiple false starts) and highly costly.
The folks on the ground simply did not believe that a protracted siege was good strategy. That means that either they thought the defenders were abjectly weak (which belies the casualties suffered and the weeks of delay when a fleet could have provided relief if it had been sent by the Venetians) or that maintaining the siege was likely to be more costly than an assault.
Saying that the Ottomans could have easily taken the place in 1391, 1394, 1411, or 1422 is getting pretty deep into alt-history. Could they have taken it then? Possibly. But doing so before having artillery (e.g. everything but 1422) was going to be far bloodier than 1453. Taking it in 1422, without having the time and funds to build the fortresses used, is even dicier as you are not going to close off the sea. At least in part, it is not that the Ottomans "just happened" to be wracked by rebellions and invasions while besieging Constantinople, rather it was having the army (and the Sultan) camped in front of the walls was dangerous for the Turks. Rebels choose to make their move during the sieges precisely because they expected the army to be diminished and slow to respond.
EUIV's endless sieges with slow, steady attrition that regularly last many months were not the norm in the EUIV era. Assaults were often undertaken not out of overwhelming strength, but out of risk of the siege becoming too costly for the besiegers.
