The whole point is, they werent considered an enemy army attacking the city. In a siege scenario, Constantinople is nigh impossible to take. This was far from a siege scenario.
???
I wonder what your definition of 'enemy army' is. The Byzantines
despised the Crusader army. They threw refuse at Alexios IV when the Crusaders presented him on a galley to them, beseeching them to surrender the city to him. They were historically suspicious of foreign armies in their environs (see Choniates' account of Isaakios II's mishandling of Barbarossa's army, a much less belligerent force whom he harried and hampered so utterly that he nearly caused them to turn on Constantinople). The pilgrims complained of the chilly reception they got when they tried to enter the city (pre-conquest) to marvel at the holy relics. Even Alexios IV, who owed everything to them, was telling the Crusaders to stuff their demands and get out by December 1203. Hell, Mourtzophlos took power mostly by playing on the populace's distrust and hatred for the big, dirty, diseased, hungry, violent ruffians camped across the Golden Horn.
Yes, of course the Crusader army is going to be a different animal from a gigantic force of, I dunno, Persians or Huns or whatevers. For one thing, it wasn't that big an army. Gregory Bell's best estimate of the numbers of the Crusading army that rendezvoused at Venice in 1202, based on Venetian records, is between is between 14,500-18,250. Of that, a substantial number were women, children, the elderly, and the sickly whom the papal representative had to command to go home. Further unknown numbers died of disease in the months before the army shipped out, and then some more at Zara. After the disaster at Zara, at least another thousand deserted. Simon de Montfort quit in disgust and took his host with him. Villehardouin claims that the number that deserted the Crusader army at Venice or Zara was greater than the number that arrived in Constantinople. By the sack in April 1204, I'd be impressed if the Crusaders had a force of 10,000. Hell, the Varangian Guard in the city by itself numbered about 6,000, according to Donald Queller and Thomas Madden. The Crusaders were partly dependent on Alexios IV's goodwill to feed them for much of their stay in Galata. A bigger and better organized army would be in a different position.
Bell, Gregory. "Unintended Interruption: The Interruption of the Fourth Crusade at Venice and its Consequences", Journal of Medieval Military History, Volume 6, 2008.