Isn't it just to do with how typical news reporting works?
A man stabbing a man at home, who are known to each other, is not an unusual event. It happens all the time and large numbers of these attacks don't really make the news. It's generally considered low interest by news reporting (or so I was taught in sociology at college a decade ago...)
A politically motivated, terror stabbing doesn't happen very often and so is of high interest.
In this case, it seems Essa Suleiman appears to have first commited the first, low news interest, type of murder then followed by the second high news interest type of murder.
It makes sense that in this case there would be some reporting on the first murder but a focus on the second and third. It entirely fits with how reporting is generally done on murder. Lots of murders never make the press (or go beyond basic local news reporting). I've certainly known one that never went beyond a single story in the local press on the culmination of the trial and sentencing.