There is no explanation for why the UA exists either. It "just does".
I'm sorry, why is the UA a better theory?
A Universal Constant happens to be a little more convincing than hypothetical puller particles riding out of the earth and pulling me towards its surface.
It's also a little more convincing than the entire fabric of space-time "bending" for some reason (it obviously has nothing to do with mass due to the photon example).
A Universal Constant is more convincing because we already know that things can accelerate, and that a mechanical upwards acceleration can keep things pinned to the earth. The elements for a mechanism for "gravity" is already in place. Something just needs to push the earth upwards. We already know that things can push and accelerate matter (an explosion, for example). We already know that it's physically possible with our current understanding.
With those "other theories" entirely odd and new branches of physics need to be created for anything to be possible. Puller particles? No evidence what-so-ever. Bending of space? Unexplained and ridiculous.