Three people being stabbed by one perpetrator is more unusual than three stabbings committed by three different people in three separate incidents. It's no wonder that it's made the news regardless of circumstances. Also I can't speak for Manchester but practically every London stabbing is reported in the media at the moment, and if there's a stabbing in my area it's generally reported too even if not in great detail.
It is being treated as a terrorist attack now so it appears that it was indeed one. I hope that the three who have been wounded will recover. What a horrific thing to go through.