I can't speak for the UK press, but Sandy Hook was front-page news for months here in the US of A. It absolutely was NOT swept under the rug. There were angry debates about what could or should be done, with one side accusing the other side of not giving a damn about children because they wouldn't ban AR-15s, and the other side accusing the first side that they don't give a damn about dead children because they wouldn't support armed security in schools.
It's been the single biggest topic in the US news and the US policy debate scene for the past three months. I don't know where you got the idea that Americans were ignoring it, but that's simply NOT TRUE.
As for "retribution," the perpetrator of the Boston terror bombing is at large, possibly planning another attack. Adam Lanza, on the other hand, killed himself. So there was nothing left to be done in terms of "bringing the killer to justice." Of course there was no manhunt after Sandy Hook, because the killer was dead at the scene.