Why does the HN headline here drop the original's important word: 'why'?

Original (emphasis mine): >> Stanford Medicine study shows why mRNA-based COVID-19 vaccines can cause myocarditis

Transcribed to an HN headline here: >> Stanford Medicine study shows mRNA-based COVID-19 vaccines can cause myocarditis

A significant difference, no?

Why?

----

After reading the article at Stanford, the word I'd choose is 'how' as in 'how vaccines can cause myocarditis'.