Basic units don't simply have an "alive" and "dead" state, with perhaps a weakened state in between - rather, they have multiple sections, each with two sets of Hit Points and internal components that can be damaged once the outer armor is stripped away, impairing a 'Mech's fighting ability in a number of ways.
The second is that the game is based heavily around Subsystem Damage rather than Critical Existence Failure.
After three centuries of exile, the descendants of the SLDF, now calling themselves "The Clans", launched a Blitzkrieg invasion of the Inner Sphere.
Following the will of Aleksandr Kerensky and armed with advanced Battlemechs, weapons, and Battle Armor, their intent was to end the still-ongoing Succession Wars by force.
This level of detail is why even a basic four-on-four skirmish can take hours to play out. The game has literally of mech chassis, many with dozens of variants a piece, but players are encouraged to customize their units.
A player might swap out his large engine for a smaller engine to exchange speed for more armor, or change a heavy autocannon to a particle cannon to increase range and reduce ammunition strain at the cost of overall firepower.