Just three reasons among others:

1. Joins (real server-side joins) - so you can have many-to-many relations in your data (and nested arrays are a poor answer to this problem)

2. Awesome query language - No strings to concatenate, no strings to escape, no JSON objects with special keys, just plain JavaScript/Python/Ruby/etc.

3. Schemaless - Faster to prototype things, to adapt to a third party data changing.