That's because Tiger ships on a DVD and a good number of Macs out there that are capable of running Tiger only have CD drives.
Apple does have a program in which customers can pay $10 and send in their installation DVD and then get CDs of Tiger sent back via U.S. mail. Information about the program is posted on Apple's Web site.
Tiger is slated to go on sale April 29. It works on Macs that have a G3, G4 or G5 chip, a built-in FireWire port, at least 256MB of memory and 3GB of available drive space.