Well, here's my take on that...
A tarball (tar.gz) is universal. Anyone with a compiler can use a tarball, and just about any distro comes with a compiler (at least an option to install one). RPM on the other hand is very specific, your system must support RPM's; worse, they are (usually) precompiled for architecture lower than what you've got so they don't perform optimally on quite a few systems.
So why would a developer go to all the trouble to make:
A slow package, precompiled for fewer distros, which causes all kinds of dependency issues, and in the end only pleases about 1/3 it's users?
It doesn't make sense for EVERYONE to make RPM's. Rather it's often times the distro's themselves who will produce the RPM's and then the developers will link these on their website simply for ease of the end user.
That's my take on the whole "why tarball and not RPM" thing