The shank is a tough piece of meat, with a lot of connective tissues. Therefore, they need to be slow-roasted or braised (as we do in this recipe) for an extended period of time to get that luxuriously tender result.
One of the most amazing things about lamb shanks is that they are almost impossible to overcook. If you leave them in the oven for an extra hour or so they will still be melt-in-the-mouth tender when you pull them out. On a side note though - once you've removed the foil during the cooking then don't leave them in longer than directed. If so you'll end up with an evaporated sauce and a caramelised brown sludge instead of rich silky gravy.