Why are the campaigns of the European knights against the Turks called crusades?

Because the Seljuk Turks captured the Holy Land and Jerusalem, where the main shrines of Christians were located. Also, eyewitnesses reported about the persecution and oppression of Christians in the lands conquered by Muslims. Popes started talking about the threat to all Christianity. In this context, the first crusade took place in the 11th century.
At the symbolic level, the campaigns were called cross, because their participants sewed crosses on their clothes, that is, they carried it on themselves, like Christ.