The A380 brakes and spoilers are deemed sufficient enough for the braking needs of the A380. The two inboard thrust reversers were added at the request of the FAA which felt that thrust reversers had to be fitted onboad the a/c.
The exclusion of the thrust reversers on the outboard engines reduces the chances of FOD damage on these engines as they hang out of the runway onto the grass patches.
Not to mention weight and maintenance savings.