In Windows 8, the name for a lot of features in Windows 7 has changed. For example, Windows Anytime Upgrade is now called Add Features to Windows 8. Disk Defragmenter has now been changed to Optimize Drives. Not really sure why the name changes were necessary, but if you’re looking for something in Windows 8 and can’t find it, it’s probably because the name has changed.
Look for more information : http://helpdeskgeek.com/windows-8/what-happened-to-disk-defragmenter-in-windows-8/