Subotic although far less talented than Hummels, is IMO, a better, more reliable defender.
There's a bit of 'star factor' as well. People might cite not seeing that much Bundesliga to argue against Subotic's quality, but they'll have no bother accepting Hummels because he's more of a household name, plays for a top nation, has some extra footballing ability. People who watch Borussia regularly know that Subotic brings the calm and technical attributes to the side.
Hummels brings the leadership.