Coaches are often imperfect beings when it comes to doling out playing time. Loyalty to certain veterans can get in the way of proper decisions as can the attempt to justify a team's bad contract.
Strewn about the league are guys who play well and could provide even more value with bigger minutes. There seems to be this fallacy where a promoted player will suffer a huge production drop-off. A lot of these guys are still good players, even if bigger minutes sap some of their quality.