1. It's the most flexible yet easily configured static site generator I've used to date
2. It's reliable. The docs are well maintained and, mirabile dictu, the real behavior of the system matches its documented behavior
3. It's stable. Lots of new features are getting added, but (so far) upgrades don't break things
4. It has good tooling for editors like vscode
5. Many of my current projects take the form of "big data pipeline, followed by intermittent job to rebuild a static site and push it live"; astro fits very nicely in this role
Caveat: I use exactly none of Astro's backend features. I understand why they've added them but, for me, it's all about static site generation.
About the only thing I don't like is the weirdness of the boundary between astro components and framework components. I get it, but still. It's something to carefully consider when building each new astro site.