As far as I know, the word "holocaust" means basically "animal sacrifice" and before Shoah was used only in that context. Why on Earth somebody thought that it should be a good name for genocide? Isn't it a little bit demeaning?
As a starting point for discussion, Holocaust actually has one of two meanings.
Both of these definitions are quite apt. The first purely secular and the second religious and depicting the innocence of those killed.
Holocaust as a term used for genocide was also not confined only to the attempted extermination of the Jews. It was used by Woodrow Wilson in 1925 in reference to the Armenian genocide.
Holocaust was used for the extermination of the Jews as early as December 5, 1942 in the News Chronicle of London. This, however, was not using it as a name for the event, but a descriptive term.
In 1957, the Yad Vashem Studies used the term as a name with the article "the" in front of it. This might be the first use of the term as a proper noun.