  Because you have a PERC 5.   Each RAID controller, when installed, provides a way to access its own BIOS during POST, and every RAID controller is different. Adaptec Controllers:  CTRL-APERC 2 Integrated (Si)PERC 3 Integrated (Si/Di) LSI/AMI Controllers:  CTRL-MPERC 2 Adapter (SC,DC)PERC 3 Adapter (SC,DC,QC)PERC 4 All (4/4e, SC, DC, QC) LSI Controllers (low-end):  CTRL-CSAS 5 (/iR,/i,/E)SAS 6 (/iR,/E) LSI Controllers (high-end):  CTRL-RPERC 5/i,5/EPERC 6/i,6/EPERC 7 (S-series, H-series)
I purchased a surplus PowerEdge 2900 that had no disks in it.  I also purchased two 146 GB SAS drives to get started.  The server has the PowerEdge Expandable RAID Controller version 5.0.1.  I plugged the drives into the server and powered up.  I never got a prompt for ctrl-m (to enter the RAID configuration tool).  After trying a couple more times, I did a ctrl-R (which is the first prompt that comes up during the boot up.  This brought me into the PERC 5/i Integrated Bios Configuration Utility 1.02-007.  It showed under Controller 0, "No Configuration!"  Experimenting around in this utility it appeared that there was a foreign configuration, which I deleted.  I was then able to create a virtual disk (VD) which is RAID-1 consisting of both 146GB SAS drives.  I saved the configuration and tried the reboot (with the installation CD for Windows Server 2008 R2 inserted and the CD-ROM selected as the first boot device), but the system never tried to read the CD.  Moreover, I still never got the ctrl-M prompt.  The "boot up" ended with the message, "No boot device available." I just realized that the installation media is DVD and it looks like the PowerEdge 2900 has a CD-ROM drive.  Any suggestions on how to get around this?  Also, why do I still not get the ctrl-M prompt during boot up?  
Trouble Configuring Disks in PowerEdge 2900