Light Cavalry lacks the powerful charge of heavy cavalry, so even though they were numerous, they were momentarily stalled by the obstruction of the Pikemen. The Demon Clan's line infantry seized this opportunity, immediately raising their muskets and firing at them.
As previously mentioned, the horses of Kyushu are exceptionally tall, almost all exceeding two meters, making the riders on horseback very prominent targets, allowing the line infantry to shoot at the cavalry over the heads of their comrades in front.
For these cavalrymen to avoid this, the best strategy would be to dismount and engage in melee combat. However, once they dismounted, they would completely lose the mobility that characterizes cavalry, leaving them powerless against the ferociously advancing enemy.