In the same way s3 is different to a dropbox, and a car being different to a bike.

Can't tell if you're ragebaiting here, but I'm very confused by this question because they support an entirely different set of features, and if you use both it's painfully obvious how they're different.

Docker is built for running services, distribution is part of that, but it's core is that you can pull an image and run random service on your machine packaged with all the right libraries, network them into your machine in the way you like so it can access the right things, constrain it's resources, and create our own image based on it.

Snap/Flatpak is built to distribute applications, sandboxing being a core part of it, with applications wanting to integrate into distribution mechanics such as audio, URL handling, taking screenshots, ...