Crusaders were called "Latins" by some narrative sources and Mediaeval historians.
Medieval sources: Albertus Aquensis, Willermus Tyrensis
Why was this descriptive term used?
Crusaders are sometimes called "Latins" because they were of the Roman Catholic Church and used Latin as their liturgical and scholarly language.
The names Byzantine and Latin were not contemporaneous terms. They were invented much later by historians seeking to differentiate between the classical period of the Roman Empire, the medieval period of the Eastern Roman Empire, and the late medieval Latin Empire, all of which called themselves "Roman." The term Latin has been used because the crusaders (Franks, Venetians, and other westerners) were Roman Catholic and used Latin as their liturgical and scholarly language. It is used in contrast to the Eastern Orthodox locals who used Greek in both liturgy and common speech. - Latin Empire (Wikipedia)
If you're asking why the crusaders are sometimes referred to as "Latin", I believe it's to distinguish them from the followers of the "Eastern" or "Orthodox" Christian Church.