We have a lot of experience in JavaScript, writing our services in NodeJS allows developers to transition to the back end without any friction, without having to learn a new language. There is also the option to write services in TypeScript, which adds an expressive type layer. The semi-shared ecosystem between front and back end is nice as well, though specifically NodeJS libraries sometimes suffer in quality, compared to other major languages.

As for why we didn't pick the other languages, most of it comes down to "personal preference" and historically grown code bases, but let's do some post-hoc deduction:

Go is a practical choice, reasonably easy to learn, but until we find performance issues with our NodeJS stack, there is simply no reason to switch. The benefits of using NodeJS so far outweigh those of picking Go. This might change in the future.

PHP is a language we're still using in big parts of our system, and are still sometimes writing new code in. Modern PHP has fixed some of its issues, and probably has the fastest development cycle time, but it suffers around modelling complex asynchronous tasks, and (on a personal note) lack of support for writing in a functional style.

We don't use Python, Elixir or Ruby, mostly because of personal preference and for historic reasons.

Rust, though I personally love and use it in my projects, would require us to specifically hire for that, as the learning curve is quite steep. Its web ecosystem is OK by now (see https://www.arewewebyet.org/), but in my opinion, it is still no where near that of the other web languages. In other words, we are not willing to pay the price for playing this innovation card.

Haskell, as with Rust, I personally adore, but is simply too esoteric for us. There are problem domains where it shines, ours is not one of them.

We changed to Python instead of Java to have the back-end processing in the same language as our data analysis module. In addition, Python has a lot of libraries for data-processing. We intend to use Flask for our back-end web development. Flask is a simple, straight-forward framework for our purposes. Flask also has a large community which is beneficial to the development process.

A major part of our project includes visualizing the data through graphs and charts. We chose to use d3.js since it provides a wide selection of well-designed graphics and animations. As a library, it is also easy to use and be included in our UI. JavaScript which our team has experience with was also selected to integrate graphics from d3.js into the UI, as well as to integrate the UI with the backend system. Along with JavaScript, HTML5 and CSS 3 are also selected mostly for styling and formatting the webpage. These three languages are widely used which means that more support will be available, making the implementation process easier.