tl;dr: We needed a modern IDE with high productivity features.
When I first started working on a project which used XQuery I didn't know the language at all. Just like the rest of the team. Step by step, by reading more about it, finding solutions to our problems on stackoverflow and generally using it, we started getting to know it. Most of the codebase in the project was Java. It seemed natural to use a good IDE for Java. Most senior people in the project had really positive experience with IntelliJ IDEA and they shared their knowledge and infectious enthusiasm with the rest of the team and taught us how to use it efficiently.
Still there was that part of the codebase that was written in XQuery. We tried out existing IDEs (Eclipse with XQDT, Oxygen) that did support the implementation that we were using (Saxon) and decided that they were not giving us the required support.