I know the answer to that, it's because a) they are not as armed as we, b) their native lands were not Founded in Liberty like ours, c) their history is one of one of being oppressed and being oppressors, and d) the Socialist rot took hold in their post-war environment so much easier primarily because they've always been more contaminated by the nearby Marxist scum that arose in Germany and reached it's Bolshevik nirvana with Lenin's Soviet Union.
I know you probably have read on the subject, the Venona intercepts and such.
The Brits and many in France and across Europe between WWI & WWII were easily exploited by Lenin's brand of Communism. I mean who wouldn't side with the people vs the oppressive imperialist capitalist state, eh? Being in nations that were little changed from their Monarchical past and still had all the trappings of aristocracy and privilege probably made recruiting at Cambridge and Oxford way too easy. Old Boys would only chortle over so and so being a "Red", made it possible for Philby, Maclean & Burgess to rise unimpeded. Boys will be boys.
And not to spare blame, America at the time of the rise of unions was heavily exploited by COMINTERN, they would expel card-carrying members on occasion, but only when they were outed. The rest of the first line cadre of card carrying commies marched on, as did their fellow travelers the non-Party Comrades and the Useful Idiots making up the exploitable socialist-leaning rabble. (For a classic useful idiot, read up on Eleanor Roosevelt, Hitlery Clintoon's personal hero)
Stalin was reading Churchill's & FDR's mail practically verbatim. Harry Hopkins, (CCC) Card Carrying Commie! Lauchlin Currie, FDR's Admin Asst, CCC! Harry Dexter White, Asst Treas Scty & 1st head of the IMF, non-Party Commie! Maurice Halperin, Duncan Lee, Julius Joseph & Donald Wheeler at OSS, Commies! Bill Weisband, USA Signals (Arlington House) the Commie who warned the Soviets and had Venona shut down. The Los Alamos spies - The Rosenberg's, Alger Hiss, Klaus Fuchs, Theodore Hall, David Greenglass, Harry Gold) They had people in bureaucracies everywhere, agencies, industries, journalists. All reporting to NKVD/KGB legal and illegal Rezidentura and their operatives.
It's no wonder Stalin had the drop on everybody in shaping post-war Europe and consolidating it!
And while the Soviet system may have collapsed from its own inability to provide for its own citizens who no longer bought the commie bullsh*t, what they spread into the West merged with the naive war-weary populace and became the Euro-Socialist crap that helped usher in the Progressive-filled bed-wetting Muzzie-appeasing pathetic sacks of crap we know and loath today.
OK, that screed went a bit long...sorry.
Y'all get the general idea...