Probably because the sport is called [association football] of which soccer is one of the short forms (from the British) same as football, and is generally used to distinguish itself in places where other sports from the same football category are prevalent.
FIFA is an association that governs various association football variants including futsal. Using acronym expansion on FIFA as a canonical source for what to call any particular variant of association football would be misguided.
Probably because the sport is called [association football] of which soccer is one of the short forms (from the British) same as football, and is generally used to distinguish itself in places where other sports from the same football category are prevalent.
FIFA is an association that governs various association football variants including futsal. Using acronym expansion on FIFA as a canonical source for what to call any particular variant of association football would be misguided.