I am unfortunately unable to give you an exact answer, but I can direct you to earlier posts and a brief summary based off sources which I've read.
That said, your question requires a bit of reframing to get an adequate answer. I would word it as: "Why did the Nazis treat E. Europeans so poorly as opposed to those from islands in the British Channel?" The reason is Nazi ideology, which claimed that Slavs (and of course Jews) were a subhuman race worthy at best of slavery for German settlers and at worst total annihilation. It was the Nazis' full intention (from Hitler's own words no less!) to turn Eastern Europe into their own version of the American West - that is, cleared of its native peoples, with those unhappy survivors meant only for slave labor. Indeed, Nazi armies in the East fully believed that they were waging a racial war of annihilation and acted accordingly. It's for this reason that we get the horrifying images from the Soviet film Come and See, most infamous of which being >!when dozens of Jews and Slavs are locked into a church and scorched alive!<. There was no similar level of racial animus for Brits or French. I'll have to stop here, but I've left some places where you can learn more.
Here are two excellent responses by u/commiespaceinvader to related questions:
Raul Hilberg's monumental The Destruction of the European Jews remains a foundational text, and I'd highly recommend reading all three volumes rather than the abridged version. It's upsetting but rewarding study.