There are no WHM/cPanel option on some of the VPS package because the package specification does not meet the minimum requirement to run WHM/cPanel.
In order to meet the minimum requirement, a server or VPS need to be:
- Processor: 266 MHz processor
- Memory: 512 MB RAM (1 GB recommended when hosting many accounts)
- Disk Space: 10GB hard disk
To run WHM/cPanel, you need a higher VPS package.
http://www.cpanel.net/products/cpanelwhm/system-requirements.html