the plane thing was a lot more protracted, we weren't even sure it was terrorism for a while and it's not like plane crashes in themselves are that rare. several terrorist attacks right in the middle of a busy western city in the space of an hour, taking hostage and killing mostly young people on a fun night out, was more unprecedented and chilling - a lot of people were watching it unfold in real time too which made it more immediate. plus, idk that this applies to america so much, but a lot of people here know victims or, like myself, know people who know victims; it's less removed from our sphere of relevance than a lot of terrorist attacks are. the fact that they targetted such innocuous public gatherings in probably the closest capital city outside of the UK has kind of hit home with the british public how easily it could happen to them and their loved ones.
i mean, as i implied earlier i do think a lot of people do arbitrarily pick and choose what things to get upset/outraged about, what causes to get behind etc, and do so in really self-serving attention-whoring ways, but in this case the extra level of interest/investment is pretty understandable.