I think Diderot was a Deist only through inertia.
People used to think that there couldn't really be any empty space. I'm not talking about air and other physical things, but just no empty space at all. This substance called Aether was thought up. People went to great lengths to prove it! The modern website, Esoteric Science asks, "How could light be a wave without something to wave?"1
No one has ever been able to prove the existence of Aether.2 No one needs Aether to use any of the functional theories we have about the world. Flip the light switch and on come the lights, even without proving aether.
The human mind wants there to be something there where aether ought to be. Some of us want it so badly that even though we don't need it and we can't prove it, we suspect everyone else is just wrong and it is really there.
I believe Diderot was a Deist because of our need to put things in big empty places. We don't know exactly how the universe came into being. We have more ideas now than he did in his time, but we still don't have still don't have incontrovertible proof, if there is such a thing as incontrovertible truth.
Deism didn't catch on for the same reasons aether faded away from the majority of scientific thought. It's just filling a hole, just a variable waiting to be defined. Many modern thinkers define that variable with either a well defined Creator or with a scientific theory.
1. "Aether," Esoteric Science, http://www.esotericscience.com/Aether.aspx.
2. "Aether Theory," University of Arkansas, http://comp.uark.edu/~davewall/Molly/Aether Theory.htm.