This question of mine got closed as off-topic, in spite of receiving way more upvotes than downvotes and also receiving a detailed answer. Why did it get closed?
The fact that a question is answered does not mean that it is on-topic or should not be closed.
The fact that people upvote a question does not mean that it is on-topic or should not be closed.
A question which does not show evidence of research can be closed by three people, since research is necessary and lack of it is a reason for closure. This much is actually in the closure message: "Please include the research you’ve done", complete with bold text and a link.
You haven't shown that "the English word foundation comes from the Latin word fundatio": you have merely asserted that, even though it may actually be true. However, in providing corroboration for that assertion, you would probably have found a step in the derivation where an o was introduced. That would actually answer your question.
The fact that people have been kind to you and done that research for you does not mean that your question cannot or should not be closed.