Why shape matters more than text
The MUTCD's most useful design choice is also its quietest one: the octagon belongs to STOP and nothing else. That means a driver doesn't have to read the word — the outline alone tells them what to do. The same logic applies to the inverted triangle (yield) and the diamond (warning).
Most countries that signed the 1968 Vienna Convention on Road Signs and Signals adopted the octagonal STOP. The US (which is not a Vienna signatory) had already used the octagon since the 1920s. Canada, the UK, the EU, India, Australia, Russia, and South Korea all use it.
Japan kept its inverted-triangle design from the pre-Vienna era. It's still red, still “STOP” in legal effect, but the shape is exactly the one that elsewhere usually means yield — which is why so many foreign drivers in Japan roll through them. Japan revisited the question in the 2010s and chose to keep the triangle largely because changing every sign nationwide would be expensive and confuse local drivers more than it'd help foreigners.