Because not only is it emotive (and they love emotive language to get you to click), it's also just an objectively fantastic word for a headline in that it's very concise and helps headlines fit on a single line.
Headline space is limited, so it's easier to go with "X slams Y over Z" as opposed to "X criticises Y over Z" or "X denounces Y over Z" or "X castigates Y over Z"
It's annoying how much it's seen. But I get why they do it.