Top reasons:
- local first files stored in markdown format (somewhat future-proof); this also allows a Git repo sync
- multiple ways of syncing Vaults (their term for a workspace); can store Vaults on iCloud, Dropbox, or with Obsidian's Sync service which is E2E encrypted
- rich plugin ecosystem
- can style pages with CSS and run various code, like javascript, from inside the editor
- Graph view and Canvas are extraordinarily useful visualizations of your data
- supportive and prolific community, from their forums, to YouTube creators and bloggers; simply search "Obsidian intro" or "Obsidian tips" or "Moving to Obsidian from ___" for inspiration