Perhaps you did an upgrade or installed Xubuntu or some such version of
Ubuntu first. As of 10.10 Software Centre has taken over from Synaptic and
Gdebi which no longer come as default applications. When you do a fresh
installation you get what is on the disk and when you do an upgrade you get
whatever is on your computer that is upgradeable. They are two different
situations. I never upgrade and I always install Kubuntu and add GNOME
without Mono later.

This applies only to Ubuntu and not Kubuntu etc. which use other package
managers (Kpackagekit or Synaptic). You can still install both of course,
but Ubuntu is pushing SC strongly. Look for ths trend to continue as
Canonical tries to separate Ubuntu from the pack. They have a vision and
change is coming.