1st season is a full of episodes that ratchet up the tension, culminating in a finale that had whiplash-inducing climax/ anti-climaxes. Then they set about topping it in S2, and they managed it pretty well : some episodes of S2 were a lot of thrill and sudden action while some others were bordering on meh; but all that did lead to an explosive finale again. Really well done.
After that, S3 onwards they just couldn't get it going. My take is that their material was for 2 seasons, and the rest was just to milk money.