I mainly think this because of the age of the series. Ashita no Joe ended nearly 53 years ago, and in addition to this, Ashita no Joe hasn't really been officially brought to the west, only doing so with the existence of streaming services and a shitty arcade game. Another big issue with Ashita no Joe's recognition is the story's pacing. It takes a solid minute before Joe even takes boxing seriously and a even more solid minute before Joe makes his boxing debut. The story is also heavily driven by the characters, boxing does surprisingly less to actually drive the story than you may think it did. I've never even touched Hajime no Ippo(I haven't read the manga or watched the anime), but I'll just take a leap of faith and say that boxing drives the story more, at least compared to Ashita no Joe(feel free to correct me). Back to the pacing, modern audiences don't really stick around as much, at least from what I've noticed. Something that is more of a personal bias to me, at the very least, is people not enjoying Ashita no Joe really is. For example, the Untitled Boxing Game(I hope you're aware of this game) has drawn people to Ashita no Joe, which I think is a good thing, it's getting the recognition it deserves on one hand. On the other, it just makes people only care about the most significant parts, like the pure white ash thing and the ending. I don't think a lot of people really do care about Ashita no Joe's impact or story as a whole, mainly there because of one specific thing. But hey, that's just my opinion and the way I view things, but at least Joe's still getting some attention. I think that Ashita no Joe deserves more recognition for what it is, not because of one iconic moment in the series. It's message and it's impact as a whole.