In addition to Monona's valid observations, I think it is useful to remember just where the system came from.  Yes, many countries had signed on before the US got serious about the matter, but the US was represented in the development of the system from the beginning.   Before there was GHS, however, there was the UN Recommendations for the Transportation of Dangerous Goods, which trickled down through the IATA, ICAO, IMO, international agreements, and national regulations, and which the world had been harmonizing on for several decades.  This system has the numbering of 1 (most hazardous) through 3 (least hazardous, although still hazardous). The Committee of Experts that developed and periodically revises the UN Recommendations was expanded in number and scope for the specific purpose of developing and maintaining the GHS, and their work is clearly based on the kernel provided by the then-existing UN Recommendations, which are still in use for transportation of dangerous goods (hazardous materials).

 

When GHS was first published, I recall considerable publication about there having been intense discussion of whether the hazard numbering should be 0-4 (least to most) or 1-4 (most to least). But as the basis of the regime was a long established 1-3 (most to least) no one should be surprised at that scheme being adopted. The rest of the world was certainly much more familiar with that sequence than with the NFPA sequence.