No. Software developers make software and package maintainers decide to
include it or not. There is much Linux software on Sourceforge and other
places that is not in the Ubuntu repositories. Obviously it is in the best
interests of promoting your software to have it in the repos.

Not having it in the repos need not stop developers, however. They can
package it to meet the specifications of various distributions and then have
debs or rpms on their site. In Ubuntu's case they could make a PPA or for
all distributions they could add a source to your sources file.

The problem is in the number of distributions and package managers. Another
problem is the way that Linux handles dependencies. Generally you can't have
multiple instances of a dependency since their is no registry file to track
them as in Windows. Shared dependencies reduce space, but often lead to
dependency conflicts that are difficult or impossible to resolve, forcing
the user to choose between two applications. The repositories mitigate this
by ensuring the dependencies are of the same version. If you stick with the
sources maintained by the developer you seldom run into dependency problems,
but if you add to the sources and install outside packages then you increase
the risk of dependency problems.