The general dissatisfaction with the end results of the Crusades -- thousands dead, lands and economies ruined -- vs. what the Church had promised for its followers also had an effect. After crusading far and wide, many soldiers deserted to start new lives. Even religious fervor couldn't last forever, and when it began to die down, the Mission died down as well.
Lots of factors are involved in this, but crucially we are talking about logistics and the difficulties there are in trying to mobilise and organise an armed force made up of many different nations and then trying to keep them supplied with food from Europe all the way to Jerusalem. Obviously, this involves massive practical challenges.
Some historians indicate the Children's Crusades (1212) of Stephen and Nicolas mark the end of the effectiveness of the Crusades. The Pope would not bless these crusades, neither would he stop them. The speculation is that the Pope hoped the children would shame the kingdoms and empires of Europe into mounting another proper crusade as money and enthusiasm were waning. Another deterrent to effectiveness was the fall of Acre in 1291. It was the last Christian city to fall.
Another factor that we need to consider is the military genius of Muslims. For example, Saladin (1138 – 1193) was a very able military leader. Moreover, when you fight the same foe over and over again, you start getting used their type of warfare. For this reason, you are able to prepare better.
It would be difficult even today to mount a military campaign so far from one's base of operations. Surely it was even more difficult then. The fact that previous crusades had not achieved lasting, long-term results must also have sapped some of the enthusiasm from later crusaders.
Poor planning and leadership were other reasons. France's King Louis IX set out on two of the later Crusades that ended in failure, and his poor health eventually resulted in his death. His successor, England's future king, Edward Longshanks, also failed to mount a successful invasion.
Europe was changing a lot at that time. As Europe went through stages, there were times when the Church was very powerful and basically controlled everything. The Church was a governing structure.
There were other factors as well. One was the decline in the political and moral authority of the Catholic Church, particularly towards the end of this time period. This time period covers, for example, the Great Schism in which there were two competing popes. Since the Crusades were a Church-led movement, the decline in the prestige of the Church naturally led to a decline in the efficacy of the Crusades as fewer people cared enough to go on Crusades called by the Church.
Part of it has to do with a lack of unity on the part of the crusaders, both between western and eastern Christians and among western Christians themselves. More importantly, they lacked sea power, which made supply and reinforcement difficult. Finally, the Crusaders could never amass a large enough force to conquer and hold territory in the Levant in the face of a determined resistance.
quite simply due to more organisation of societies, along with the ability to make agreements more peacefully. And finally, it would be silly and immature to attempt a crusade. It can easily grow out of proportion in the modern age due to social networking