Because it undergoes uniform circular motion, and in uniform circular motion, the velocity is changing but the speed is constant, meaning only direction is changing
Remember there are always 3 cases. When theta (angle between velocity of charge and magnetic field) is 0, meaning field is parallel, F=0. When theta is 90 degrees, which is this case it undergoes uniform circular motion (because the force by RHPR is always perpendicular to motion). When theta is between 0 and 90, helical motion will occur