Because he is a hireling (hoti misthōtos estin). And only that, without the shepherd heart that loves the sheep. Reason given for the conduct of the hireling after the parenthesis about the wolf.
And careth not for the sheep (kai ou melei autōi peri tōn probatōn). Literally, “and it is no care to him about the sheep.” This use of the impersonal melei (present active indicative) is quite common, as in Mat 22:16. But God does care (1Pe 5:7).