* Atom was hackable from the ground-up. I use a VS Code extension called Customize UI that allows me to write custom CSS, but Atom supported custom CSS as a first-class feature.
* Nostalgia: Atom was the first text editor I used, and so it has a certain friendliness than other editors can't match.
* Atom One Dark and Atom One Light are the best pair of dark and light code themes I've ever used. I use ports of these themes in other editors, but you can't beat them in their original context.
* I don't love VS Code's whole design language (straight-thin lines and sharp angles, except for some corners, which are curved). But that's probably very much tinged by my other points.
* Atom, of course, matched VS Code on language support and plugin support at one point. You could find extensions for just about any language. (Something that wasn't ever true for Sublime or Textmate, for example.)
* Being truly open source (I would describe VS Code as more open-core).
* Not being the most popular editor. I love supporting the underdog in my software choices as a way of encouraging innovation and variety in the ecosystem. So if Microsoft makes VS Code so good that no editor can complete with it, I would use VS Code (VSCodium), but I would not be happy about it.
All that said; Atom's dead. Since the Microsoft acquisition, development has stalled. Atom's gotten increasingly buggy and many many packages were abandoned. As I understand it, there are architectural issues that make it very difficult to fork or maintain. Atom is upstream of Electron (not downstream of it), for example, since Electron was originally extracted from Atom's code, so that makes integrating Electron updates difficult. CoffeeScript (which a lot of Atom is written in) never really caught on. You have a bunch of existing packages, but as mentioned, most of them are unmaintained, so you'd have to rewrite them and it's not clear which ones. Tree-sitter was originally developed for use with Atom, but after most popular programming languages had packages, and they never got rewritten with tree-sitter.
I've been following Lapce. I think they're doing interesting work. Right now the editor is too buggy for me to consider as a daily driver. I've been using Emacs for the past couple of weeks. It exceeds Atom's hackability, but there are still a lot of rough edges in the UI, for someone used to GUI editors.
I didn't mean to write this much. Atom is obviously something that I care about and that I've been thinking about ever since the acquisition.
TL;DR: Some people don't like VS Code, as a personal preference, and other options for GUI text editors are good.