M12 x 1.25 is a standard ISO thread, it's just not very common. Most likely this was chosen over the more common M12 x 1.75 because it provides better grip at the expense of less strength. Clearly the one thing you wouldn't want is for the top to get loose when you're in the middle of some turning.
Standardisation has always been a contentious issue. Obviously customers want it, but for suppliers it opens the door to all sorts of issues (like people starting a debate with a lathe manufacturer where in reality the issue is with the maker of the chuck). And by the way: The only three countries who have never signed up to the metric system are Myanmar, Liberia and the United States, and that's one of the main reasons many commonly used standards in the anglophile world are still in imperial measurements.