1. Privacy. You can roll your own syncing (or use iCloud, Dropbox, etc.), without the notes being stored on the note editors' servers, which is a huge win over Roam, Notion, Evernote, etc. After the Evernote fiasco from a few years ago (where they considered reading your notes for ML model training and got massive pushback), I value future-proof solutions that won't become a liability in 10 years if the company providing the note taking software gets desperate.
2. Markdown. Speaking of future-proofing, Markdown is as close as it gets to having interoperability with the notes. Obsidian is at its core just a Markdown file editor, which means your notes are stored as plaintext and easy to export. There is a bit of Obsidian-flavored syntax (e.g. bi-directional links [[...]]), but these are becoming standard in note-taking. Many note-taking apps claim export functionality, but at the end of the day they're not incentivized to give you your data in a format that will work with other editors.
3. Executing on features that have become indispensable for note-taking, and personal knowledge management specifically: bi-directional linking, block-embeds, query-embeds, unlinked mentions, graph view, custom CSS, note aliases, markdown diagrams (via mermaid), and a few others.
4. Offline support. If there's any kind of login or sync required to access your notes on your personal device, that's a dealbreaker for me. This seems to have been a regression lately in the latest batch of note-taking apps.
My entire personal knowledge base was in Evernote for a few years, and now happily migrated to Obsidian. The graph view is just a magical way to explore the knowledge that you've stumbled upon over the years.
The team gives away so much value in the core app for free. If you enjoy their product, consider supporting via the Catalyst plan: https://obsidian.md/pricing