That’s a very good question, and you’re not alone, because it’s something that historians of the crusades wonder as well!

In fact there was some interest in Spain even before the First Crusade. In 1085 Alfonso VI of Leon captured Toledo from the Almoravids, and in 1087 there was a French expedition to Tudela in northern Spain. The French didn’t accomplish much, but many of the knights on the Tudela expedition participated in the crusade a few years later.

When Urban II became pope in 1088, he was very concerned with restoring the city of Tarragona in Almoravid territory south of Barcelona. Urban talked about indulgences and forgiveness of sins and described the expedition in terms of a pilgrimage, just like he would for the First Crusade a few years later.

There were other military expeditions against Muslims before the First Crusade. Also in 1087, at the same time as the siege of Tudela, Genoa and Pisa attacked Mahdia in Tunisia. The Normans in southern Italy began conquering Sicily in the 1060s, and by 1091 they had conquered the whole island as well as Malta. The Normans were supported by the popes, including Urban, who gave them a papal banner to fly during their conquests. He also offered indulgences and other benefits, like in Spain.

The Normans were encouraged to go on the First Crusade, but the Spanish were not. This is probably because Sicily was already fully conquered by then, but Spanish knights still had work to do in Spain. But they were promised the same material and spiritual benefits as if they were going to Jerusalem.

“As far as the liberation of people was concerned, therefore, Urban regarded the new crusade to the East as part of a wider movement of Christian liberation and did not distinguish it from the Spanish Reconquest.” (Riley-Smith, pg. 20)

Spain was a secondary target for people who were on their way to Jerusalem by sea. Around 1110, the King of Norway helped the Spanish conquer the Balearic Islands, and in 1147, at the beginning of the Second Crusade, English and German fleets conquered Lisbon. In 1189, during the Third Crusade, some other cities along the coast of Spain and Portugal were also captured by English fleets. By the 13th century crusading and the Reconquista in Spain were pretty much indistinguishable. Everyone knew that was part of the crusading movement too.

The problem is that medieval people did not have a word for “crusade” - they simply called it a pilgrimage, or a journey, and “the journey” meant the pilgrimage route to Jerusalem. It was different from a regular pilgrimage because it was actually a military expedition, but crusaders were expected to dress and act like pilgrims and visit the holy sites in Jerusalem once they got there. So, in that sense, it was separate from the military expeditions in Spain. There is a major pilgrimage site in Spain as well, Santiago de Compostela, but Santiago was already under Christian control by the 11th century. Unlike Jerusalem, there was no need to liberate it.

Spain also didn't have the same psychological significance for medieval Christians. Jerusalem was supposed to be the centre of the entire universe and the place where, for Christians, God temporarily became a human. If Muslims ruled some or all of Spain, medieval Christians weren't bothered by that as much as they were with Jerusalem.

Since they didn't have a word for "crusade", it's really modern historians who created the distinction between the crusades and the Reconquista. Just out of tradition or convenience, they are usually still treated separately today. But at the time, medieval people definitely thought of them as part of the same movement. However, Spain by itself did not inspire people to go off and fight a far-away war, they needed the extra incentive of Jerusalem first.

Sources:

H.E.J. Cowdrey, “The Mahdia Campaign of 1087”, in The English Historical Review 92 (January 1977), pp. 1-29

Jonathan Riley-Smith, The First Crusade and the Idea of Crusading (University of Pennsylvania Press, 1986)

Bernard F. Reilly, The Kingdom of León-Castilla under King Alfonso VI, 1065-1109 (Princeton University Press, 1988)

Paul Chevedden, “’A Crusade from the First': The Norman Conquest of Islamic Sicily, 1060–1091", in Al-Masaq 22 (July 2010), pp. 191-225

Joseph F. O'Callaghan, Reconquest and Crusade in Medieval Spain, University of Pennsylvania Press, 2013