Any thoughts on why the delta wings are naturally more stable in pitch, so they don't require any tail surface? What's the physics behind this?
This is a very good question, as it shows some of the misunderstandings regarding "tailless" aircraft. With deltas, one can consider them as blended wing/tails.
The function of a horizontal tail surface is pitch control. The back of a delta is certainly large enough to do this. So much so, that now one can design a forward canard to go with it. This plane should pass the horizontal drop test (nose must go down).
Notice that birds, wanting more straight wing lift, "unblend" a little and also have a fan tail. Delta's do not generate as much lift as a comparable straight wing.
So designers can work with what they need, particularly with desired speed range. Faster, the wings can pull in a little, as with the FB-111 swing wing.