Most probably because your are running with a standard VGA graphics driver. Install the required graphics card driver to enable suspend.

You need to enable hibernation under power options. See requirements for more details.

First verify that suspend or hibernate are working. Use Remote Desktop to connect to your server. Then click on Start->Shutdown and select suspend (or hibernate). Should this fail, try to find the reason. Have a look at eventlog, check for blocking services or drivers. Also verify that external drives (USB or eSATA) can be used with standby. Sometimes a driver update is required to make that work. 

When a manual initiated standby is working check the runtime diagram in Lights-Out. All sources which keep your server active are recorded their.

This is either a driver problem or a mis configured network card. Update your drivers and verify your network card settings.

Some machines require an additional parameter in boot.ini to limit the amount of usable memory: /MAXMEM=4096 or /MAXMEM=4095. 

See Microsoft KB888575.

Frequently asked questions