Of course, its not as serious as 3 deaths in the USA.
It was an incident in the US…. at a mass-participation international sports event. Its news-worthiness comes partly from the terror angle (and the speculation over a terror attack with no apparent obvious claim or point being made) and partly from the fact the people were taking part in, and spectating at the event ( and therefore potentially amongst the victims) were from all over the world. So its prominence in the UK news schedule reflects the fact there will be a significant number of UK viewers who’ll be following the story because they know friends and family of theirs will have been close to the event.
Being a sporting event it was also widely covered by cameras – so theres news to actually show. Theres less news you can make out of an event in a comparitively closed country with a government who’re not terribly open or honest in their dealings with the international media and a lack of a available facts, footage or eye witness testimony.