Probably because, in spite of there being many, many more white men in this country than black/brown/Muslim men, and plenty of those white men are undoubtedly hostile to Islam and frequently quite racist, it's a very rare occurrence for a white man to target and attack black and brown Muslims with a bladed weapon. Proportionally speaking, it's almost unheard of.
Whereas proportionally speaking, it's not anywhere near as rare to find black or brown men frequently from Muslim backgrounds (but not always) and frequently from illegal immigrant/asylum seeker backgrounds attacking people with bladed weapons. And being involved in crime generally. Proportionally speaking it's not that rare, unfortunately.
Also, the other pretty obvious reason is that the white men doing the attacking are probably British by birth and as horrible as they might be, there is very little point in rioting to demand that the government deports them, is there?
Whereas, sorry to point out the obvious, but every time an immigrant who wasn't born here does something like this, (especially when it turns out that they are an illegal immigrant or someone who has sought asylum and then repaid our kindness by living a life of violence and crime) certain questions are bound to be asked.
If the government was doing its job properly then very often those people would not have been here in the first place and therefore their victims would not have been attacked. It's frustration and anger over that and the depressing number of times it keeps happening that causes protests and in extreme cases like the Southport child murders, civil unrest.