"Why do we mourn for Britain – but not Nigeria or Yemen?"

It's very simple, Western countries, especially the Anglo-American ones are a bunch of hypocrites.

Have you also noticed how Western media reserves the word Terrorism for acts of violence against white victims? When non-white victims are the targets, Terrorism is put in quotation marks like this: "Terrorism"; otherwise they describe it as a "mass killing" or "attacks" and provide a cursory root-cause analysis? 

Subliminal messaging: attacks on white victims are abhorrent crimes while against non-whites, they are sorta explainable / justified.