This follows the similar naming convention Apple has followed for previous versions of Mac that contained updates to their predecessor like Lion / Mountain Lion and Leopard / Snow Leopard.
In fact, its name is really significant as El Capitan is the name of one of the most popular rock formations at Yosemite national park and it signifies that this version of Mac had more under the hood changes to the previous version that is Mac OS X Yosemite. It built upon the design changes of its predecessor: Mac OS X Yosemite. It included under the hood performance improvements, new natural language processing and AI support for spotlight searches and much more. In this update, Apple added metal graphics processing methods which allowed for faster graphical performance of apps in MacOS.
Mac OS X El Capitan was a major update introduced by Apple.
