It is more that Coppola is relatively old for the level and his combination of tools really do not translate at higher levels. He is fast but really not that fast. Not sure his defense is seen as anything special. He does not have an especially high walk rate either for an older player. No power as mentioned.
There are reasons the Phillies have promoted him slowly. Could he be the rare older player that develops into a major league bench piece? Sure. But the way the Phillies have handled him means they don't really believe in the tools. Probably see him as a Ben Revere who is not as fast or as good defensively.