Well we've had only ever had two nuclear holocausts and they were almost contemporaneous with the genocide of the Jews by the Nazis. It's perfectly understandable that the word would have been used at the time (and stuck) for the destruction of Hiroshima and Nagasaki. I agree that the capitalisation of the H draws a line between the genocide of the Jews and other forms of mass slaughter, but you can't capitalise it in speech unless you say clumsy things such as "the Holocaust with a capital H." I can't agree that you can use the word in any context you like these days without invoking thoughts of the Nazi atrocity. Perhaps we should all have been using "Shoah" from the get-go, but the reason we haven't has everything to do with the inexorable evolution of language, not with deliberate co-opting, appropriation or cornering of the market. Always remember that Hebrew prefers Shoah.