Why? Because you start from the base of a working game, and incrementally add the specifics of your desired gameplay. In other engines you'll start with more or less a blank slate, and not only is that intimidating for newcomers, but it also comes with a large delay between building something, and being able to interact with the result.
The Blzzard map editors are incredibly powerful. They've spawned multiple entire genres (Tower Defense and MOBAs both originated as custom maps).
They have an accesible scripting system that is kind of a blend between a visual and a text-based programming language. They have an extensive content library built in, so you don't have to learn 3D modelling before you get started on gameplay...
