Can I ask why? I've noticed other developers frequently only provide their in-development builds on Steam, and any other source only receives stable releases. Does it take too long to roll them out to anywhere else?
Well for us, there is no other channel that would work for this. The only other channel is the DRM-free download, and that has no beta branch system at all. And it would be torturous to ask players to re-download over and over every day - we'd be constantly getting reports from old versions that people forgot to update. Steam makes sure everyone is up to date.
Finally, for the purposes of testing, there's just no need for more people than Steam gives access.