They are rewarded for winning. Rewarded with re-election, promotions, etc.
So they have incentives to shield info that makes winning less likely. Like the info about alternative suspects mention in this motion.

They knew back in 1999 that someone else threatened and had motive to kill the Lee. Their case against Adnan relied heavily on the threat and motive (based on the testimony of Jay).
So disclosing this alternate suspect would be very damaging to their case against Adnan.

What are the consequences of a Brady violation for a prosecutor? And how often a Brady violations pursued? I can't find anything with quick googling.