First and foremost, people of the Middle Ages didn't call these expeditions "crusades" during the time they were performed. The name and the numeration came later, possibly during the 1800s' historiography.
Secondly, the effort to continue the "war effort" on such a large geographical scale was born of many circumstances and factors which did indeed occur simultaneously but not al all times worked in correct unison.

The eponymous First Crusade (1095-1099), the one which resulted in the greatest success of all the military expeditions of that period, had come to fruition in an era where the papacy born of pope Gregory VII had violently bashed horns with its previous clergy, drawn mostly from aristocratic families of Europe and thus very intertwined at a political level, and with Holy Roman Emperor Henry IV for the right of nominating said bishops. This reformed Church had gained significant moral prestige if compared with its predecessor of the previous century, and was thus able to rally most of the nobles and lords of the Christian Western Europe to make an armed pilgrimage towards Jerusalem.
This last part is VERY important: people went on a pilgrimage there already at the times of Charlemagne, when caliph Harun al-Rashid signed treaties to allow pilgrims safe passage. This changed at the turn of the XI century, when Turkic Muslim tribes grouped around the Seljuk dynasty started conquering the Near East and climaxing in the Battle of Manzikert (1071) when Turkic forces gravely defeated the Byzantine Empire and settling parts of Anatolia. The leader of this last action was sultan Muhammad bin Dawud Chagri, also known more famously as Alp Arslan.

Turks seemed to be less tolerant of pilgrims rather than Arabs, causing many troubles for the Christians travelling towards Jerusalem. Also, it appears that they disrupted or weakened several trade routes connecting the Western ports of the Mediterranean (such as Genoa, Amalfi and Venice). Plus, they heavily weakened the Byzantine Empire which had almost singlehandedly faced off against Muslim conquest since the VII-VIII centuries. This last battle may have prompted basileus Alexios I Komnenos to seek the help of pope Urbanus II to contain this advance. It also was meant to stop the wars with the Normans across the Balkans. In this particular timeframe, kings in Europe were very weak and most of the political and decisional power laid within the many counts and lords, especially in France (which someone has called the "superpower of the Middle Ages") being the largest of the kingdoms. Petty lords and bannal powers were abundant and were these powerful individuals and their allies which embarked on this armed pilgrimage towards the Middle East.
For the West, it was an immense logistical and financial strain for everyone involved. This, however, is for both the rich Tancred of Hauteville, descendant of the Norman lords of Siciliy who became prince of Galilee after the conquest, to more humble men which embarked on the journey for the same reason.

In the same moment, the Middle Eastern polities and leaders were far less united than what they had been centuries earlier when they pushed the Byzantines away from the Levant (I'm not as well informed on this topic as on the Western side of the story, so take this section with a pinch of salt), possibly due to the aftermath of the Turkish conquest of the region. The influx of combatants from the West appears to have been less effectively contested by local powers than the successive - failed - expeditions of later crusades.
Also, the prospect of a new area of the world with new land and new opportunities (as many people, especially knights, going on the crusade thought of staying there rather than coming back home) prompted many scions of noble families whose properties were either been already so split up among descendants to no longer being able to be inherited, or which had lost their wealth, to join the expeditions.
This last part is also important. People were setting off for the Holy Land at all times, not only during those dates and years we are familiar with. Some scholars have proposed the idea that from 1095 to 1291 (as far as the Holy Land crusades are concerned, not to mention the Northern ones in the Balkans and Prussia) there had been a constant influx of men and resources from West towards the Near East, on all levels of organization.

In the end, the Western lords and powers did not have enough strenght to keep the land they had conquered, as we know. Not that there had not been great efforts to maintain them: military knightly orders, the extensive fortifications of the kingdoms of Outremer and the other great expeditions were all efforts in this direction, but ultimately the political climate of Europe and the material costs of these actions were unbearable for such a titanic struggle over such a long distance.
The greater goal in mind was to secure the Holy Land for Christendom, in order to protect the Holy Places for the pilgrims which wanted access. I am not convinced that it was a simple economic, "pre-colonial" war of annexation, neither it was a fanatical effort of genocidal proportions to destroy the Islamic faith (as there is no source regarding this ludicrous claim). I am convinced that it was indeed a practical, material problem (the aforementioned weakened Byzantine empire, the expansion of the Turks in mainland Europe, distruption of eastward trade), but also that many people which joined the expeditions believed in their call to protect the pilgrims and their right to go and see the places of their religion. Of course, Muslims had the same right and purpose but that's another discussion entirely as tolerance was not the main problem of many people of this period.

But I want to conclude with a note towards this direction. When Holy Roman Emperor Frederick II joined the Sixth Crusade in 1228, he obtained control of Jerusalem without a war. Speaking six languages, among which, arabic, he came to an agreement with Ayyubid sultan Al-Kamil, which returned Bethlehem, Nazareth and Jerusalem to the crusader realm for ten years, and inside Jerusalem, the Dome of the Rock and the Aqsa Mosque were to be retained by the Muslims as all of their homes and property. Christians were able to visit and pray at the Holy Places. I wish to add that Frederick had been excommunicated since he had not left earlier in 1225.
Lastly, in 1219 a four-weeks truce during the siege of Damietta by the aforementioned sultan Al-Kamil. In this timeframe, when, according to Jacques de Vitry and Tommaso da Celano, saint Francis walked to the sultan's camp and was allowed to stay and converse with the sultan's scholars and philosophers, coming back unharmed at the crusaders' camp with good wishes by the sultan.

Sources:
Barbero, A., Benedette Guerre. Crociate e Jihad, Laterza, Bari, 2009;