Explanation: Medieval European logistics were... primitive. And the logistics of the international and ad-hoc undertaking of the Catholic Crusades was doubly so. When called to arms, feudal polities generally did not have an actual system for feeding and supplying their troops - they just made arrangements as they went. Who paid for what was often a matter for argument, depending on the feudal contract, and part of why medieval European polities struggled to raise large armies or endure long campaigns.
In-line with this total lack of logistical organization, or organization in general, there are accounts during the Crusades of poor troops stealing food out of hunger during periods of garrison (not crisis) and being hanged for it - because their lords either could not or would not pay for their supplies, and there was no centralized system to ensure distribution of food - or even its availability. Even if most men in the army could afford food, even if the supply was not short in the local market, if you didn't have money or connections, you were shit out of luck.
In the Crusades, matters were doubly borked since the primary recruiting method was not "We need X amount of armed recruits from Y region, and your duty as His Majesty's vassal is to see it done, or else pay a fine"; but rather, "If you are willing, GOD NEEDS YOU TO RETAKE THE HOLY LAND!" Even for those who had a clear overlord who was crusading, and requested their presence, the logistical preparations were "You should take along about two years' worth of money to buy supplies" rather than any organization of supply lines or depots who would be expecting to receive a sudden influx of mouths in a time of near-subsistence survival.
Not only did this mean that responsibility for many of the 'soldiers' coming along was unclear, but also that their usefulness in military operations was doubtful to begin with. Notably, a force of some 20,000-60,000 'Crusaders', the People's Crusade, was entirely obliterated by ~5,000 Muslim troops during the First Crusade.
... the Muslim polities they fought with were generally more organized. Surely the failure of 8 out of 9 Crusades is unrelated to this fact.