It's not that Walnut is dark, so much as it's usually a much cooler colour than most other timbers, which makes it look shadowy and darker. That's one reason it's considered such a special and individual wood. In the US an African timber called Ovangkol or Shedua is popular as a substitute for European Walnut, that's even cooler toned still.
Your photos illustrate this well, the Walnut stains are knocking out the red and yellow tones in the underlying timber. In fact Oak is quite cool toned too, not as cool as Walnut but moving in that direction. If you'd have shown comparisons against say Mahogany or American Cherry stains then the difference would have been even starker.
Furniture fashions come and go, and a cooler look is currently on trend, but the Victorians for example thought warmer was better, so they'd frequently re-finish older Walnut furniture with garnet shellac to add warming red tones.