i have a mac pro with os x 10.6.8 why can't install a mavericks on him ?

To install Mavericks, you need one of these Macs:

• iMac (Mid-2007 or later)
• MacBook (13-inch Aluminum, Late 2008), (13-inch, Early 2009 or later)
• MacBook Pro (13-inch, Mid-2009 or later),
• MacBook Pro (15-inch or 17-inch, Mid/Late 2007 or later)
• MacBook Air (Late 2008 or later)
• Mac mini (Early 2009 or later)
• Mac Pro (Early 2008 or later)
• Xserve (Early 2009)

Your Mac also needs:

• OS X Mountain Lion, Lion, or Snow Leopard v10.6.8 already installed
• 2 GB or more of memory
• 8 GB or more of available space

Apple probably didn't develop the drivers to support your hardware in Mavericks. No one here can give you a definitive answer since we don't work for Apple.

Your Mac Pro is too old. See the list above.

Because it can be a model that is "Later/newer" than Early '08.

Lion (10.7)

http://support.apple.com/kb/ht4949

http://store.apple.com/us/product/D6106Z/A/os-x-lion

You can. I have a macpro 1,1 running Mavericks.