Honestly, I think it’s a few factors:
1) Harry has paranoid schizophrenia, and it can be really ugly. Tumblr fandoms usually recognize PTSD, depression, or some anxiety disorders.
2) Harry’s not usually depicted as attractive. And when he is, it’s usually not with the paranoid schizophrenia he had in the comics. The Raimi films used alcoholism, the USM version didn’t really use mental illness, and The Amazing Spider-Man 2 was terrible about Harry’s character. This ties into shipping as well; Parksborn doesn’t seem as popular as canon ships or as Spideypool.
3) Harry can be an ass sometimes, and his mental illness really can’t be romanticized. Props to the writers for doing this, but he can lash out and be difficult to be around in the comics when he’s suffering from a breakdown.
Honestly, I feel like it’s a shame. Harry Osborn is one of the few comic characters who actually gets treated for his mental illness, and lives a decent life while still having a few ups and downs. And it’s not a stereotypical comics mental illness either. It’s a shame he doesn’t get more love.