Well, for one, the old Reddit interface's appeal is fairly limited in general. Most people use apps anymore, I feel like in general desktop users are not catered to even in default interfaces. Of people who do use the desktop, more of them are likely to be new(er) Reddit users who probably find the Lemmy/Piefed default UI to be superior to Reddit - but the pool of people who prefer old Reddit is dwindling, which doesn't usually lead to continuing support of things.

As it stands, few Lemmy instances have it and in any case it feels pretty fragile. It has broken a couple times and I think been fixed, but this has led instances to drop it altogether (dbzer0 used to have it, for example, but the lead said it was too much of a hassle and its devs weren't responding to his questions).

I'm fairly worried that 1.0 is gonna break it, tbh. It never gets any updates that I have noticed, and its functionality is a bit all over the place.