The dropdown is handled by the Bootstrap js. http://getbootstrap.com/javascript/#dropdowns
Changing the behavior of the dropdown would not be possible with pure CSS. You’d need to dequeue the compiled Bootstrap js and add your own hover-based dropdown. Doing so would lose a lot of the other Bootstrap components unless you re-compiled a version of the Bootstrap js without the dropdown and enqueued that.
The reason Bootstrap does not use the hover state for dropdowns is for mobile performance — on mobile devices, there is no hover state, there is only a single tap action.