Simple answer. As Bootstrap has been progressing, then went for a new sort of style since version 3. They've gone for a more of a flat UI style.
Joomla still has Bootstrap 2.3.2 and therefore uses the older style which isn't flat. Updating the Bootstrap files on your server will be an extremely big task as so many things we're changes, for example the grid class names. I'm not sure when Bootstrap 3 will be used but I assume it might come with Joomla 4.x