It's the Maggie Smith Effect

Call Waldo an old so-and-so, but Downton Abbey, and its attendant craze, bored him silly. The series is nothing but Dynasty without the shoulder pads and the fight in the lily pond.

What made it such a fad? A writer at The American Scholar thinks he's onto them:

Things are a lot more fun with motorcars and decent plumbing, but I don’t think that’s really the heart of it. The Edwardians have history on their side—which is to say, on the other side. Because it’s almost never only the Edwardians. We start in that Indian summer of Empire, but then comes the war, and the crash, and the slump—the whole procession of decline and fall. Upstairs, Downstairs started in 1903 but ended in 1930. Downton Abbey, which is basically Upstairs, Downstairs with better production values, saved time by starting in 1912. Even when the story stops before the guns of August, a sense of what’s to come hangs over the proceedings. The Edwardians possess that priceless dramatic asset: the pathos of dying aristocracies.