The thing is that after the First Crusade most of the knights went home. They had done their religious duty and it was time to return to Europe and the wife. The ones who stayed behind wanted a piece of the action and for the most part were second or third sons of nobles who had nothing to return to. The Crusader states were always short of manpower. Part to the reason for the creation of the Templars and Hospitaliers was to provide a stable source of warriors to defend the Holy Land. In fact the Crusader states were never a high priority for the kings and nobles of Europe who were always busy fighting their fellow Christians at home.
Again, this is all correct. But is kind of begs the question as to why the crusades were preached in the first place. I'm no Medieval historian, but as I recall, there was no immediate threat to pilgrims in the Christian sites. Perhaps Manzikert was viewed as a significant change, but I doubt it. Urban saw an opportunity to lump together religious fervor, the problem of population pressures (in his view), the problem (genuine) of violent knights engaging in street brawls, and a Muslim enemy that everyone wanted "rolled back."
The Church provided pilgrim status for all who went, further increasing the chances that people would join up, because the Church would guarantee their lands. I'm not saying the Crusade was necessarily wrong---just that to portray it as ONLY a religious episode misses much of what was going on in Europe itself, and does not explain the timing. As you point out, other crusades were disasters.