Example: the national costume of Malaysia for men is the Baju Melayu. The current Malaysian Prime Minister is Muhyiddin Yassin. Googling for images of him, we find lots of photos of him in a suit and tie, but comparatively few of him in a Baju Melayu. Why? It seems especially appropriate for him to do so at venues where he is representing the country, e.g. at international meetings, but no - usually every male leader at these meetings wears suits and ties. Female leaders wear more varied clothing, but still nobody wears their country's traditional costume.
A first guess is that traditional costumes are usually impractical to wear, but that doesn't seem like the case - e.g. the Tangzhuang, Sarafan and Kilt all look very wearable.