I mean there's several levels here:
- 310 people being killed for their religious beliefs in a third world country is less of a news item than an old cathedral burning down with no casualties in a first world country, because too many people only care about what's happening in their own back yards. (There was another ethnically & religiously motivated attack last month in Mali that killed 160 which also went mostly unnoticed in media.)
- Politicians don't want to name the victims as Christians, possibly out of fear that this will encourage more "alt-right" people to carry out retaliatory attacks like the one in New Zealand. (But this reasoning doesn't make much sense either given that the most right-wing American Christians tend to be virulently anti-Catholic, and just ends up downplaying the religious motivations of the murderers.)
- The alleged culprits are from an organisation that—like all other wahhabi-influenced Sunni Muslim paramilitary organisations—most likely receives funding from a US & European ally, Saudi Arabia, which US & European leaders don't want to alienate, so very little action can be expected in response.
I interpreted a lot of the frustration as stemming from the first point, because the tone-deaf American responses had not yet happened at the time & there was also not yet any claim of responsibility or knowledge of the perpetrators.