Because there is no standard packaging format between distributions, the best people can do is release a deb that may or may not work on either debian or ubuntu. Or an RPM that may or may not work on RH/Fedora. Or a tar.gz that will contain sometimes source code, sometimes a binary, and if it has source code there's no clear way to compile it, if it has a binary there's no clear way to install it.
So instead, curl sh.
It's pretty ridiculous that it's still a problem, tbh. What we need is somebody with the right political clout at either RH or Debian to sit down with devs of the other, come up with a decently generic solution that fits both systems and a backwards compatibility policy. It doesn't have to be perfect nor the best, it just has to be good enough to support the use cases of most Linux distros.
Once both Debian and Fedora support it, Ubuntu (and its many flavors) and Red Hat will follow. And if it's a generic enough system, with a pluggable-enough backwards compatibility policy, it's not unlikely for it to be adopted in more minor distros. If it's sold correctly, distros will gladly adopt it - we've all been waiting for this a long time.