Comment 2 for bug 1164122

This is a default decision made by the Ubuntu team. I can't seem to find anything that justifies this decision but to get around it you'll need to change a configuration file and then recompile your kernel. This isn't as hard as it seems and there are resources online to help you out.

To recompile, follow the directions in this article https://help.ubuntu.com/community/Kernel/Compile
Make sure you change the configuration line listed in this article before compiling though http://askubuntu.com/questions/165507/how-to-change-maximum-number-of-cores

Hope that helps.