Because for instance Node.js has several mature frameworks (in terms of stability) for doing HTTP servers such as Express. But Express comes with just the most fundamental building blocks and it's up to you to pick and choose what you need to complete the stack for yourself.
Express is what I have mostly used and loved it, it deals with a lot of the lower level stuff while I was learning my way around developing web apps and APIs. But now with each new project I am faced with dealing with user auth, emails, billing, migrations and what have you, I'd love to transition to something that allows me to focus mostly on business logic.
Other than that, I have heard really nice things about Elixir/Phoenix. Everything is a stream so it does depart a bit from the others. Python, PHP and Ruby are all more traditional setups.