> No one has conclusively determined why yet
Ask immigrants or the children of immigrants (like me).
Western Europe are just extremely insular and passive aggressive and will gladly bury their head in the sand regarding their own racism.
Anglo countries at least tried to tackle racism by introducing education about racism (eg. Civil Rights Movement in US, Aboriginal Rights Movement in Australia, First Nation's Movement and racial quotas in Canada, the anti-Skinhead movement and the trade union lead anti-racism movement in the UK) and trying to build an identity that trascends race or ethnicity.
Similar movements happened in mainland Europe as well, but aren't taught about, so mainland Europeans remain insular in their mindset.