Unless you were being sarcastic, it's because people don't know what the word infamous means and think it means "extremely famous".

Could just be a misuse of infamous but could just as well be intended to refer to the fact that Man and Mankind mean the same. You need an article in front to transform "One small step for Man" into "One small step for a man" to refer to Neil himself stepping.

"One small step for a man" is what he actually said, I think, or so I've heard. Apparently the "a" was lost due to radio interference.

Wikipedia (see elsewhere for link) has good coverage of that. "A" was intended to be said, but when humans say lines like that it is common to miss a word here and there. There is no way to know for sure if he said it and the technology of the time didn't pick it up, or if he misstated his own quote.

It's like flammable and inflammable. They mean the same thing and nobody knows why we have different words for it.

At least, this is my head-canon.

Exactly! This has been my own head canon too since like decades! I was actually surprised to read in this thread that "infamous" is a negative interpretation of famous which seems like a revisionist and recent interpretation. The English language also evolves through the ages and so do the meanings and interpretations.