What reasons, in your opinion, did not allow the crusaders to hold onto the holy land?

The main reason the Crusaders were driven out of the Holy Land is fragmentation. Each count considered himself an independent ruler. If it was to his advantage, he could easily conclude an alliance with the Muslims and go to war against a crusader neighbor. The lack of a unified command led to the defeat of the crusader army. Also, thanks to the actions of some crusaders, the local population, to put it mildly, did not like the power of the European knights, considering them barbarians. There was also no need to wait for targeted and tangible help from Europe, reinforcements arrived in small detachments.