Great observation!

Once upon a time, scholars postulated that the lack of kings on the First Crusade was part of a deliberate papal strategy to maintain control over the expedition. More recent attention to the workings of medieval political culture, however, show that Urban would have been well aware that the higher the prestige of the crusade leaders, the more support (financial and manpower) from lower nobles he would accrue. Indeed, we know he reached out to some of the kings and at least one reached back. However, the political situation of Europe and the spiritual situation of individual kings made those specific monarchs' participation undesirable or impossible.

First, the kings of the Iberian peninsula were engaged in their own war against Islam. Throughout the Crusades, popes actively discouraged Spanish knights from traveling to the Holy Land; they offered extra indulgences for French knights to go fight in Iberia instead of the Near East. The Iberian kings, from the pope's point of view, needed to fight their own fight.

It might have been a bit awkward to invite the count of Sicily, Roger, whose army at the time was made up of significant numbers of Muslims. (Latin Christians certainly made and broke alliances freely with Muslim governors once on crusade, but the nature of the crusader army as a pilgrimage would have made European Muslims' participation awkward.)

The Norman conquerors in England were militarily busy, but they arranged with Urban to send more than adequate financial support instead.

Henry IV was still Holy Roman Emperor, and he was not on the best of terms with the papacy since that time he, you know, kicked the pope out of Rome.

And so it came down to France. The French monarchy at the time was weaker than the other crowns, controlling a small strip of land around Paris, but Philip was putting forth a concerted effort to consolidate and increase his power. He certainly saw a starring role in a crusade as a great way to do it, and called several councils of his own in preparation.

But.

Philip had a little problem with the Church. See, after his wife Berthe gave him the much-desired heir, Philip decided he preferred one Bertrade de Montfort. He repudiated Berthe in 1092, but the Church was not a fan; an archbishop excommunicated him in 1094...for the first time.

Over the next years, Philip wound his way in and out of the Church/pope's graces, as he promised to leave Bertrade and return to Berthe only to run right back to his preferred partner. By the time the Crusade actually picked up steam, Philip was in no way the pious, ecclesiastically-devout leader the Church wanted for its martial pilgrimage.

So overall, the lack of kings on the First Crusade wasn't a grand plan--it was pretty much an accident of personalities. I'm not sure we can draw conclusions about the connection of kings to Crusade failure. After all, amidst all the military failures, it was a German king (and failed military crusader), Frederick II, who successfully negotiated Latin Europe's way back into the Near East for a short period.

He was, of course, excommunicated at the time.