His appearance was originally as a young man, and now that there's an anime adaptation of the part where he first shows up and a game which uses his young self as the representative one, his young appearance is likely going to be the more iconic version of the character now to people who don't know much about the series.
In addition there's a lot more art in the Joseph Joestar (young) tag than for Joseph Joestar (the one used for his old appearance), likely as a result of the above. So people searching for/drawing images of "Joseph Joestar" these days probably have the young version in mind anyway.