the original one killed people b/c humans weren't as used to it and the variant wasn't designed for that. This may sound weird, but germs don't want to kill people, they want them to live and keep the virus going until it infects another person. So a new variant comes along that moves faster but is much less deadly this outcompetes the original variant by getting there first, immunizing the person when the original variant shows up, too late. In other words, vaccines have been given way too much credit for ending the most of the death count (or maybe the vaccines influenced the history of variants, who knows)