I'd say it's to do a lot with advances in turbos. Most small engine cars have turbos now that rival anything even a decade a go. This means you can get by with much smaller engines and get similar performance.
I once had a Sunbeam Avenger which was horrible in very many ways (not least because of the build quality in English unionized car plants) and an uneven torque curve (with a peak and a not that high peak as such) was one of its problems.