What makes you think the others aren't constantly under attack? I think there's only so much bandwidth/space the average person is willing to give to "torrent site problems". That probably leaves room for only one, maybe two sites to make headlines. That's my guess at least.
TPB is "constantly under attack" only because it managed to be so resilient. They've already killed other huge torrent sites such as isohunt, demonoid and others, and it didn't take them that many tries.