There are various regulatory trade-offs between safety and cost of compliance. For instance: Cars sold as new in Canada require daytime running lights and yellow rear turn signals, due to extended periods of low visibility. While the running lights are a requirement in Sweden, they aren't in Poland or many other European countries. There's been some effort on standardizing between US and Canada since 2016, which would allow for a North American market car to solely use yellow turn signals, but progress is slow.
Which is why we probably won't see much progress in harmonizing bigger regulations. Whose regulation gets to win? This has been a five-year fight over the color of a piece of plastic. Everyone even wants to agree - the automotive industry would love a worldwide standard to reduce specialized part count, rulemakers would love to agree in order to increase the shared market size (so everyone gets the same cars), but the actual standard to be put into place calls into conflict the exact safety versus cost thresholds each government has set. Imagine if there's actual conflict.