The second movement of Prok. 7 has two strikes against it in terms of difficulty to memorize: 1) it's a 20th century work where, although it is diatonic, the tonalities are not always intuitive, 2) It's a slow movement, and slow movements are - for me at least - harder to memorize.
I find it harder to memorize a lot of 20th century music - with the exception of Ravel, Debussy.
Classical music is easy for me to memorize because of the formal design of most pieces, but slow movements are harder. Romantic era music is not much of a problem for me, with the exception of Brahms because of the thick texture of a lot of his stuff.
Bach is difficult to memorize for me.
I haven't decided whether slow movements are more difficult to memorize because they don't imbed into my tactile memory as quickly, or because they are usually technically easier to play, so you often don't have the repetitious practice that you would with the outer movement. If the latter is the case, then slow movements aren't really more difficult to memorize.