AMC was seen as the network for great shows, kind of like how HBO was several years back. It's now know that was't due to planning, but pure luck.
There may be different skill-sets at work. It may be that they really are great at choosing show concepts and picking decent creative teams. They just have a mental block when it comes to recognizing the importance of keeping those creative teams happy and stable long-term. It could also be a different set of executives choosing the shows from the ones negotiating the day-to-day expenses and year-to-year contracts.
Another explanation is that they got screwed in their negotiations over Mad Men and that the whole network is on a death spiral as a result (higher costs on one show leading to a a penny-saving mentality for other shows).