> Even so, I wonder what advantages Java has, that makes using it in that environment so overwhelmingly compelling over other languages.

Presumably not the whole thing needs to be that fast, only the part that actually handles the individual events. Java has the advantage of being an industry standard, or at least being very common in the financial sector.

So you can work in a single language while only having to write some un-idiomatic Java in places where performance is critical.