At a greater risk to aircraft and pilot. You can fire a Brimstone and hit your target from up to 20 km away, far out of range of AA and manpad missiles. A-10 cannon is line of sight. While you're shooting at it, it can be shooting back at you.
Actually, not really, since everyone who has noticed that an A-10 is rolling in on them, and they don't advertise their presence well at all, are usually running!
besides, unless the MAN-PAD hits the cockpit, the pilot will most likely pose the question “What was that noise?” as he goes home after splattering body parts all over the landscape.
What will destroy an F-16, will not much bother an A-10.