It should be noted, first of all, that the picture of the Crusades is a bit more complex than your question supposes. The First Crusade, for example, was an unmitigated success. The Crusaders captured Jerusalem in 1099, which they would continue to hold until 1187. Considering that the military objective of the Crusades was the Christian reconquest of the Holy Land, this detail should not be underestimated.

I think, when you look at the geography of the Crusades, you should not be surprised to find the Crusaders were at a severe disadvantage. The Crusaders were drawn from Western Europe, whereas their targets were located in the heartland of the Islamic World. Consider the strategic picture the Crusader States faced: these Christian outposts would have been themselves been isolated and surrounded by enemies desperate to recapture the territories they had lost. Looking through the lens of logistics and reinforcement, the odds would have been against them.

Beyond this, I would also suggest that because the Crusades were drawn from across Medieval Europe, you should not expect the Crusading forces to have had anything near the cohesiveness that we expect from professional standing armies. This problem that would have been particularly severe among Crusaders motivated primarily by the desire for glory, personal power, or wealth. Furthermore, you should factor in the split between Western Christendom and Eastern Christendom, especially in light of the Fourth Crusade, where the Venetians (long time rivals of the Byzantines) actually co-opted the Crusade, and turned the Crusaders against Constantinople itself.

The Crusades failed for a number of reasons: First,There was the incapability of eastern and western Europe working together. This was precipitated largely because of the great rift between the Western Roman church and the Eastern Orthodox or Greek Church. Consequently, the Western church had to go it alone with little or no assistance from Constantinople. Second, the Crusades also lacked the sea power needed. They were forced to go a long overland route from Germany or France through Eastern Europe and Asia minor.The complete exhaustion that this trip entailed left their armies weakened and certainly not fit for battle. Third,They were simply not enough crusaders to effectively settle and colonize the area of Palestine and Syria.They could not provide the presence that was needed to maintain and administrate a kingdom in a largely Muslim populated land. Fourth,far too often the Crusades became about achieving personal victories or political victories instead of freeing the holy land from Muslim domination. Finally,There was not an understanding, or desire to understand, the peoples that they conquered. Their mission had become solely to replace Islam with Christianity instead of allowing the two to coexist peacefully.