Very early on people were taught (in school!) that .com meant it was a business, and would likely be moderately correct about that business, .net was for networks, and .org was a organization that would very likely be truthful. Fake information would only come from .coms, and you could certainly trust .gov.
Of course, even then slashdot.org existed so the distinction wasn't really effective; later it DID become a bit of a signal because the "big three" were more expensive than .biz and friends so the latter became known for scam/spam sites.
Now it's all completely random and doesn't matter at all.
As your sibling comment mentions, people used to be taught (apparently in school, but I went to school a bit early for TLDs to be part of the curriculum) about the meaning of the TLDs.
It has never been the case that you needed to be any particular kind of organisation (or an organisation at all) to get a .org, but in the early days it did tend to be used more by non-profits and the like.
Of course, even then slashdot.org existed so the distinction wasn't really effective; later it DID become a bit of a signal because the "big three" were more expensive than .biz and friends so the latter became known for scam/spam sites.
Now it's all completely random and doesn't matter at all.