Not sure Python ever had any significant inroads. It's always been Java and C#. All other languages are pretty outlier by comparison. Golang has seen a big uptick in recent years though.
Having worked with and for many finance giants I'm speaking from my personal experience. Just like every other enterprise on the planet they tend to build their core tech around mature and easily staffed technologies. Both JP and GS are "mainly" java shops I know for a fact. Outlier projects with other languages aside do happen, but that doesn't mean it's the mainline tech. I'm not sure how you derived otherwise from the numerous java projects in their github. Also public repos are only tip of the iceberg.
