Commands are organized by how they are used. Microsoft found that people using Microsoft Office favor a core set of commands, which they tend to use over and over. Those core commands are now the most prominent.
An example is the Paste command. It's one of the most frequently used commands. Why not give it maximum exposure in the window, along with its related commands, Cut and Copy?
Frequently used commands don't have to share space anymore with a range of remotely related commands on a menu or toolbar. They're the ones that get used, and so they're the ones that are at your fingertips.
Less frequently used commands are less prominent on the Ribbon. For example, most people use Paste Special less often than they use Paste. So, to access Paste Special, you first click the arrow on Paste.