"...Marcus, being the youngest of the family had to wait the longest for his time in the cockpit. When he was finally accepted into the academy, he was amazed at how easy it was for him to take control of the hulking 'mech."
Assault chassis are utilised by the majority of the front line - they are designed to be able to give and take a significant amount of damage. They have the highest amount of Hull Integrity of the three chassis and therefore are most suited to the out and out rigors of combat.
As a pilot gains experience, they are able to pilot stronger and more robust chassis.