There's a few factors leading to that. Partly it's just Elixir and Ecto (the ORM used) being incredibly expressive.
Specifically with GraphQL it's the combination of it being self documenting, which allows UIs like GraphiQL to make it really quick to get going, and the fact you end up defining the structure of your data, rather than a few specific ways to query that data. Using that you can then query for whatever you need, wherever you need it, and you're able to compose queries really quickly.
