That's a good question indeed. Also I wonder why they picked java as the implementation language.
Date of commits shows this project is 3 years old.
It’s JetBrains who were synonymous with Java so not a surprise, if was a recent project would have been Kotlin (which this company created)
Because Youtrack itself is written in java
Likely because they are a Java shop. All the IDEs they develop use Java, so they have quite an expertise in low level optimization for this language.