Grayscale can be sort of a Do It Yourself activity. The issue is, just how dark or bright should some shade of red or green appear in grayscale? The standard Grayscale Menu converts it the way the human eye perceives brightness of colors. The Grayscale menu is from the way television cameras did it in the B&W days. It is how B&W film does it, matching the eyes response. That is the standard solution, considered "correct".

But there are other ways too, like the Channel Mixer menu, where we can specify how the various colors should be rendered in grayscale. Technically that is NOT a grayscale conversion, but instead is an editing operation, subject to whim, any way we want it to be. The difference might be considered pleasing instead of accurate.

My guess is it is the top portrait that you perceive to be dull and lacking contrast, and that the bottom architecture is full contrast with "visible blacks and whites". But if you look at those two histograms, clearly the top portrait has already been edited to seriously clip both ends, which is blacker blacks and whiter whites (the sky is white, the shadows are black). IMO, a bit more White Point, down to 230 where the data actually starts, might help it. Anyway, a little clipping could be called standard practice in many cases of grayscale.

The architecture image was not clipped, but that subject itself has more contrast, but it is NOT blacker blacks and whiter whites. Just more of it is nearly black or nearly white. Large black areas can be dramatic, esp if contrasted with solid white.

Ansel Adams modified his classic photos with drastic darkroom procedures (would compare to drastic editing in digital), and one of his basics was that grayscale seriously needs ample contrast (blacker blacks and whiter whites, and having some of both was his requirement), and he did this in ample degree, perhaps to a fault in some cases. Because Grayscale has no color to help it. But in color images, the color itself is often the contrast, and often serious clipping is detrimental to color (it changes the colors), where it can be helpful in grayscale.

See my site at https://www.scantips.com/lights/graycard.html#ansel about easy and standard methods using the Adobe Levels control to increase the contrast of grayscale images (without altering the tonal differences of colors). It is done "by eye", judging the preview. My own notion is that most Grayscale images need some degree of this contrast enhancement. Try it, you'll like it.