Some guys are just tweeners. Their skill set makes them great AHL players, but aren't good enough to be effective NHLers.
When fractions of a second can make all the difference, it doesn't take much to put a player just under the bar.
Exactly. There are bunches of these "tweener" types. They can post big production in the AHL where there's a bit more time and space, and things are a bit less structured defensively, more breakdowns to take advantage of, weaker goaltending, etc. But just don't have the physical traits or the hockey sense to capitalize consistently in the NHL...and don't have the skillset to really thrive in a lesser role further down the lineup in the NHL. They excel when they can be an offensive focal point, big minutes, central to a good PP, and handling the puck constantly to get into the flow of the game. They struggle when asked to play a more limited role, bring energy and PK/defensive elements to the NHL level, and don't really have a chance to get a real feel for the puck consistently. And they just don't have the talent and ability to push better players out of those opportunities in a good NHL Top-6.
The big difference with Conacher, is that he's stuck around North America a lot longer than many of those top "tweener" types do. So many of them ultimately end up playing over in Europe where they can be prominent scoring players and make pretty decent money in a cozier situation. But Conacher seems content to stay in the AHL and play his handful of NHL games as an injury replacement every year. It's kinda admirable, his dedication to it really.