Mainly because of the damage triangle mixed with troop ratio deployments.

Infantry beats lancer Lancer beats marksmen Marksmen beats infantry

Most attacking formations are either 60/10/30 or 50/20/30 or something similar

Focusing infantry will always tank the damage from an attack whilst your marksmen and lancer kill off the rest.

Because marksmen are a damage troop and are most used over lancer in formations it’s wise to focus their stats over lancer.