I wasn't personally involved in making this decision, I was brought onto the project after it was taken, but there was a perception that we'd be faster if we did everything as a full-stack application. In fairness, we did deliver what we set out to do so it was a success, even if there were some rough edges to deal with.
Also bear in mind that I'm only covering my front-end perspective on this. I think the back-end developers involved on the project found it much easier to work with as they were already experienced with Elixir and Phoenix. For them, a tightly-coupled front and back end meant that they could change the way things worked and it was straightforward to update a few functions in Elixir and then the HEEX template, as opposed to a decoupled setup where we'd probably have been communicating using a JSON API and changes would have had more of a barrier to adopt.
Also bear in mind that I'm only covering my front-end perspective on this. I think the back-end developers involved on the project found it much easier to work with as they were already experienced with Elixir and Phoenix. For them, a tightly-coupled front and back end meant that they could change the way things worked and it was straightforward to update a few functions in Elixir and then the HEEX template, as opposed to a decoupled setup where we'd probably have been communicating using a JSON API and changes would have had more of a barrier to adopt.