It's a license issue. Unlike most other distros, Debian strives to become a totally free operating system, as the term "free" is defined by the GNU philosophy. You are free to use some other distro if you like their policy on license issues better.
On the other hand, you can use some of the available workarounds to install Java SDK on your Debian system. The Blackdown version becomes available via apt-get if you add the following repository to /etc/apt/sources.list:
(This is for Sarge -- tweak it accordingly if you use Woody or Sid.)
To install Sun's version of Java, check out dastrike's guide:
Ooops, it seems that ftp.tux.org has repositories for sarge, sid, testing, and unstable so these will work but there's no repository for woody or stable.