The Gimp is in Ubuntu main (see https://edge.launchpad.net/ubuntu/+source/gimp )
Ubuntu wants the installation to fit on a single CD. Most other distros, including Debian, distribute on DVDs, but unless you receive a hardcopy from somewhere, it can be problematic to download if you have a slow or unstable Internet connection, and basically, on the DVD image you download lots of software you don't need or want. CDs are much more manageable, and once the system has been installed, you can carry on customizing your system.
With the growing number of drivers and kernel modules, occasionally software needs to go from the CD image, and here, as in writing, the rule is: "kill your darlings." Nobody *wants* to get rid of The Gimp (like others have said, it's a seminal application) but taking a hard look at the facts, you need to admit that The Gimp is a program you need to spend lots of time with to be able to use efficiently, and users who want it can get it faster than you can say: "in a few clicks." (Well, almost :-))
OTOH, a photo app is valuable to have on the install image when you have to win the hearts and minds of users coming from other OSes... it's all a part of fixing Ubuntu bug #1.