There isn't that much of a difference after all.
It's mostly a matter of perception because the R-7 family evolved into many different variants, so it feels like most Soviet launch systems use liquid boosters. Proton, for example, doesn't use any strap-on boosters.
The following "early ICBM" designs of the early 60's are actually pretty similar:
- R-7 with a liquid booster/sustainer layout, where the first stage (liquid strap-ons) is ignited together with the second stage (sustainer core).
- The original Atlas with its stage-and-a-half layout, where booster engines drop off and the core sustainer keeps burning.
Essentially the only difference here is that Atlas doesn't drop first-stage tankage, and that's because the light balloon tanks made that unnecessary.
Both designs were motivated mainly by concerns about reliably air-starting a bunch of big kerolox engines. It's way simpler to ignite everything on pad and then just drop stuff off, even though it's strictly less efficient as part of the second-stage tankage is already empty by the time the first stage burns out.
However, in the end, the R-7 family ended up spawning most of the Soviet space program, while the Atlas was just one of many competing designs among US launch systems.