Answer :
Because ...
-- If you ignore air resistance ... which we always do for these problems ... then there's NO horizontal force acting on the projectile.
-- Since there's no horizontal force on the projectile, its horizontal acceleration is zero.
-- Since the projectile has no horizontal acceleration, the horizontal component of its velocity is constant.
-- If you ignore air resistance ... which we always do for these problems ... then there's NO horizontal force acting on the projectile.
-- Since there's no horizontal force on the projectile, its horizontal acceleration is zero.
-- Since the projectile has no horizontal acceleration, the horizontal component of its velocity is constant.