I think huub wattbike tested chains and found out that you can run wide chains more slack which reduces friction and more than equals the narrow chain.
also most 1/8 chains are made super cheap and not up to the standard a chain could be these days (zero friction mentions the bushingless) construction. I now use the high end kmc and it runs smoother than all the japanese chains.
Chainring flex is also an issue. A chainring for narrow chains must not necessarily be thinner, but that would be another way to save weight and make production more efficient.
Australia was running 1/8th at worlds. GB also as far as I can recall. They probably have data they don’t want to share.