Difference being that links are essentially single fouling points which a belt fed system will then disgorge. With a drum you're bringing that debris into a consistent system, there are a lot more fouling points and any actual fouling is likely cumulative. Also keep in mind that a belt thrashes around during feeding, ridding itself of debris, where as once something is within a magazine it's a lot less prone to escaping. Also it's easier to pre-emptively spot a serious fouling issue on a belt than inside a magazine.James told me to post this and I thought it to be a good examination.
"Think about this a belt fed, m249 or a 240 bravo or any belt fed for that matter has the ammo exposed to the elements. Sure the ammo may get wet or may get a little dust on it, but it's no different than a belt fed only the outer most ten rounds are exposed and not nearly exposed as a belt fed. "