A lot of things conspire towards productivity.

The dev tools (build, deps, project generation) are not necessarily better than everything else but they are very good.

The paradigm of Erlang that Elixir builds on is outstanding for building reliable, efficient services. It makes trade-offs for that. Trade-offs that mostly don't have downside for your average web app.

Things that begin to get complex in many ecosystems. Low latency "live" functionality and lots of concurrency are trivial and supported from the ground up.

It is a high level dynamic language. Think abstraction-level Python or Ruby. But it has these very different trade-offs.

Docs are good. Ecosystem is not as frantic as JS.

Phoenix LiveView is particularly in service of delivering things quickly. Follows a very 80/20 approach. High abstraction level. Covers most needs. Not shy about making trade-offs to avoid complexity. Much like Erlang originally.

All of that and the fact that the Phoenix CRUD golden path is really wide and low incline. The workflow for implementing arbitrary business rules as an online database that just works is easy to reason about, and things don't get out of hand easily by just adding pages and tables to your app.