Because Russia is hands down the best operator of these types of human intelligence networks.
Not because they’re just smarter than everyone else, or have some genetic predisposition to being better, but because they’ve had to mass-produce people with this exact skill set to maintain their own empire for over a hundred years.
What we think of as Russia, is really mostly Slavic Muscovites who have stitched together the largest continuous empire on Earth and spanning many different ethnicities and races who didn’t join by choice, but by being conquered.
It just so happens that the tradecraft and skills required for own geographically vast domestic intelligence networks are pretty much the same ones necessary for this type of deep cover foreign espionage.