[SippinLean]

Because when everything is a class (even better, every selector is a single class like BEM strives for) overwriting rules is much easier because they all have the same specificity.

When you mix element selectors with class, ID, and multi-class (.foo.bar) selectors the specificity of each is different, and overwriting them means writing needlessly complicated selectors that are then in turn harder to maintain.

Bootstrap 4 goes as far as eliminating most sibling/child combinators (>, +, etc) because they add specificity. Anyone that's tried to write custom classes for list elements in Bootstrap 3 (.list-inline>li) has experienced this.

http://v4-alpha.getbootstrap.com/migration/#navs