Why is the delta wing naturally stable in pitch? 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.