*Spoilers
It is quite clearly and obviously revealed to the audience at the end as they zoom in on his sled.
I think because back then people didn’t have the capacity to rewind they might have missed it, and there was no internet, hence the legend arose.
What a great film by the way.
If you're asking why people who didn't see the film didn't know it, then I agree with you it's because there was no internet
But even today, people might see the film without knowing anything about it other than it's a good film to see. Unless someone deliberately spoils it for them, they're likely to wonder about "rosebud" for exactly as long as the film runs
(I should confess that I saw the film at a college screening, having no idea what it was even about and definitely not knowing the significance of "rosebud" to its plot)
The mythological status wasn't so much from people not knowing the mystery but rather its importance to the film itself
It has a metonymic quality ... people can't think of the film without thinking of "rosebud"
It would have been pretty hard to miss, especially on the big screen. The camera stays focused on the word "Rosebud" as the sled slowly burns.
Here's a capture of that scene: https://i.ytimg.com/vi/fr93wwtiKQM/maxresdefault.jpg