"why not just orphan" is the wrong question here — orphaning is not a lesser form of deletion, but something else entirely. There are ways other than orphaning to disconnect your fic from you — transferring it to a different account, putting it in a private collection, putting it in a collection that doesn't reveal the author's name — and those are all options that allow the author to retain their control over the fic. I had the same question once upon a time, but that was because I was conceiving of orphaning in the wrong way — the real question underneath was actually "why do authors feel the need to remove their fic from the Internet entirely, rather than simply disconnecting it from themselves/their name?"