They used an Atlas V. It was a legacy series rocket going back to the Gemini days according to one article. Obviously there have been improvements since then.It is good old American tech and it was powerful enough to get starliner to the ISS. They went with what they knew that would work! Atlas V is mature tech!
Atlas V is mature tech!
—
So mature that the Russians are not going to send any more - when the stock of engines is used up, time to more on - like to Bezos, if he ever gets his awkward contraption to work.