I struggled with making this decision, but ultimately it's all about readability for me. Hope it's not too much of a hassle for folks, but that's why this happens in a major update and not minor ones :).
1. A dash is an operator, which could get confusing when you are doing math. For example: (@grid-gutter-width * (@grid-columns - 1)).
2. As far as variables go... thisIsDoubleClickable, and_so_is_this, but-this-is-not. I suppose it's not the biggest deal in the world, but I really hate having to drag my mouse to select.
My two cents...
Sometimes I hear that people don't want to have to press shift, which is understandable considering how much we code. But if bootstrap devs are already pressing shift in order to use camelCase, I wonder why they didn't pick underscores.