What's your source for that? I'd have assumed that the reason was the engineering effort required to produce an engine the size of the American F-1, leading to

What's your source for that? I'd have assumed that the reason was the engineering effort required to produce an engine the size of the American F-1, leading to smaller engines, at which point it becomes more natural to steer with variable thrust than with gimballed engines. The Soviets didn't produce anything rivaling the F-1 until the RD-170 in the mid-80s, and that uses 4 thrust chambers instead of 1 as the F-1.