[philipkglass]

Only the Soviets were daft enough to build RTGs using Strontium 90.

The United States did too.

Oak Ridge National Laboratory technical report "Strontium-90 Heat Sources"

https://technicalreports.ornl.gov/1971/3445605716035.pdf

Introduction

Compact electrical generators powered by heat from radioisotopes have been under development in the United States since the early 1950s for space, marine, and terrestrial uses. Essentially all the generators developed for marine and terrestrial uses have been powered by 90 Sr. This report summarizes the development work done by Oak Ridge National Laboratory (ORNL), Hanford Atomic Products Operation, and Martin Company, Nuclear Division, which led to the production of 90 Sr heat sources for use in the generators.

It was a natural choice since strontium 90 is an inevitable byproduct of operating any fission reactor, and was readily available as a coproduct from weapons plutonium production reactors. Making better RTG isotopes like plutonium 238 required additional infrastructure.