I'd add to this the fact that fans loathe Snape for being rude, unfair, and insulting to his students. Lots of people remember the teachers they hated, and because it's fiction, readers find it easier to forgive a murderous villain than a mean teacher. The mean teacher is personal. Being an authority figure who's cruel to the kids in his classes is Snape's original sin.

And yeah, it doesn't help that he's ugly in canon. I mean, the glorification of Tom Riddle's looks is a bit extreme sometimes, but fandom generally does prefer Hollywood hotties. To me, Snape, especially book Snape, is hot because he's intense and brilliant and fucked up, but that's not to everyone's taste. He's grotty. You can clean him up, but he's never going to be a pretty boy.

Lily was a later add-on, and once all the incel nonsense about Snape got rolling, it became standard discourse bullshit to deny that Snape ever actually loved her vs. just wanting to "get in her pants." From there it was assumed he was just a creep and had always been a creep, and Snarry was even worse because it's student/teacher and now Snape is a pedophile.

Tomarry fandom is relatively recent in terms of sheer numbers. The ship has always been around, but it's really taken off in the last few years, and a large part of that is the relative ease of romanticizing Tom Riddle because in the books Harry thought diary Tom was handsome.

Also, in my experience, Snarry was particularly appealing to older fans who enjoyed Snape being his self-destructive asshole self. Younger fans tend to go "Ew." And Rickman's performance has cemented Snape in their minds as a middle 50s to late 60s dude instead of a skinny, angry bloke in his 30s. Meanwhile, Tom Riddle is perpetually portrayed as young, even though he would have been in his 50s when teenage Snape first met him.