Eclipse isn't portable, somebody has to compile it for Linux, it just comes down to whether you want that done by the Eclipse Foundation or Debian people.
My theory is that it is obviously because there is a way how to run Eclipse, without any issues and without the need of an officially tested debian package, so there is not huge demand.
Something like Netbeans doesn't need a debian package but Eclipse is split into lots of jar files that each depends on some shared libraries.