This is a collocation. I believe a search (say, in a corpus database) will show that it emphasizes the viewer's emotional state...generally depressed or filled with ennui.
The collocation to emphasize topography would be "flat terrain".
If a teacher were explaining a field-trip destination, "flat landscape" would be very unusual. Now, an art-history teacher might be explaining a dull painting, or a literature teacher might be quoting from a narrative about a character with severe existential doubts.
It would be interesting to compare the collocations used in all the reports on pictures from the surface of Mars.