Why is Rachmaninoff so difficult for pianists?

The first Rachmaninoff I ever heard is his Rhapsody on a Theme of Paganini. Here's a performance by Yuja Wang:

Vladimir Pleshakov has some interesting remarks on Rachmaninoff over at Quora:

We have three answers so far. They explain well the physical difficulties of large stretches that were easy and natural for Rachmaninoff’s hands. His virtuoso technique, which had no weak spots allowed him to write for himself, in the sense that it was easy and natural for him to play his own music, while other pianists had to work much harder to get the music going.

But there are other aspects of his music which make it difficult to interpret. In a subtle way at first, and in later compositions much more overtly, he was a post-romantic, a modernist, if you wish, disguised in the cloak of a Romantic. He is closer at times to being a neo-classicist and even an expressionst rather than a neo-romantic.

Playing Rachmaninoff’s music with sentimentality is musical suicide. Playing his music fully objectively is musical betrayal. His music reflects humanity. Rachmaninoff’s sources of inspiration are very diverse: pre-tonal church chant dating centuries back, highly distilled folk song, visual art, literary allusions, subtle influences of the oriental Russia, of Ravel, Faure, Gershwin, highly distilled American jazz, and other influences he never talked much about (such as family, friends, self-imposed exile).

He walked a pathway parallel, but never too close, to Arensky, Tchaikovsky, Prokofiev, Stravinsky, and more distantly, Chopin and Liszt. He never imitated, consciously or unconsciously, any of them. He was interested in ballet, in American musicals, in Stanislavsky-type theater. He loved the human voice, and it permeates like a rare fragrance all of his music - orchestral, vocal, and piano. He was also a superb conductor, and a superb orchestrator, transferring both of these skills into his piano music especially in terms of rhythm and color, but never for cheap effect. One can say that his piano music, for all of its apparent explosion of notes, is actually sparse and austere. In some ways, there is not a single superfluous note - the texture has been carefully pruned by the composer.

Finally, he was a Russian composer, without adhering slavishly to the nationalist concept and creed.

A pianist who can understand and unravel all of these numerous strands will solve the riddle of Rachmaninoff. Rachmaninoff himself felt that only two or three of his contemporaries understood his piano music correctly. He also felt that only a few orchestras and conductors were completely on the correct wave-length. Stokowski, Ormandy, the Philadelphia Orchestra were his ideals.