1.Speed of work - with a host of ready-made elements, creating layouts with Bootstrap takes much less time.
2.Flexibility - due to dynamically adjustable grid adding new elements doesn't violate the general structure.
3.Easily customizable - editing styles is accomplished by creating new CSS-rules that are executed instead of the standard ones. Plus, you don't need to use the attributes like !important.
4.Large number of templates .
5.Huge community of supporters/developers.
6.Wide range of usage - Bootstrap is used when creating themes for virtually any CMS (Magento, Joomla, WordPress or any other), including single landing page applications.
7.Great official documentation.
for more details visit: http://basicuse.net/articles/pl/textile/html_css/getting_started_with_bootstrap