Because it works and it works great. It's fast and it never crashed on me. Give me an equivalent opensource editor that works and looks like Sublime and I'll use it. No, Atom and VS Code don't cut it. They're slow, they choke on big files and eat up hundreds of megabytes of RAM. I don't need or use plugins, so I don't care if it's extensible in <insert trendy language here>.
Vim is also nice but I only use it for remote console work. It doesn't have the nice features that Subline has (PCRE, select and Ctrl+D editing) or they're clunky.
Well that's what was using on Linux before I installed Atom, although it's hardly a replacement for Sublime (which gets used on the Mac at the office for real work). Really, I've tried just about everything.
Vim is also nice but I only use it for remote console work. It doesn't have the nice features that Subline has (PCRE, select and Ctrl+D editing) or they're clunky.