A large part of the reason is the autobrake system depends on a properly functioning antiskid system.
Should the antiskid system fail, the autobrake will not arm or will disarm.
One of the parameters the antiskid system requires is a valid IRS input.
Prior to the IRS interfacing with the antiskid system, aircraft speed and deceleration signals were a function solely of wheel speed transducers in each wheel assembly sending its RPM to the A/S computer.
The problem with this is should the wheels stop spinning due to severe hydroplaning or severe runway ice, the computer might think the aircraft stopped moving.
With the IRS coupled to the antiskid and thus autobrake, these systems aren't solely dependant on the spinning wheels for spinup/speed/decelleration signals.
As a sidenote, the A300-600 uses nosewheel speed sensors for its antiskid reference speed, as well as the IRS.
You're only as good as your last departure.