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Why is Rachmaninoff's Piano Concerto No. 3 considered that difficult?

I recently watched the film [Shine](https://www.imdb.com/title/tt0117631/) (1996, by Scott Hicks). It depicts the life of the pianist David Helfgott.

As Wikipedia explains in [Shine (film)](https://en.wikipedia.org/wiki/Shine_(film)):

> In London, David studies under Dr. Cecil Parkes (John Gielgud) and enters a Concerto competition, choosing to play Sergei Rachmaninoff's enormously demanding 3rd Concerto, a piece he had attempted to learn as a young child to make his father proud.

The difficulty of the piece, together with other issues, lead to the pianist having a breakdown.

Over the film there are several references to the difficulty of the Piano Concerto No. 3. And so Wikipedia's [Piano Concerto No. 3 (Rachmaninoff)](https://en.wikipedia.org/wiki/Piano_Concerto_No._3_(Rachmaninoff)) explains:

> The work often has the reputation of being one of the most technically challenging piano concertos in the standard classical piano repertoire.

It is indeed impressive to see how it is played, and also listening to it in different plays (over Spotify) shows very difficult movements of hands and changes on rhythm.

Still, I don't have enough knowledge of piano to distinguish its difficulty in comparison to other pieces. For example, to the extremely fast performance in [Asturias](https://en.wikipedia.org/wiki/Asturias_(Leyenda)), by Isaac Albénic. For this: **what are the main reasons of the Piano Concerto No. 3 by Rachmaninoff to be considered one of the most difficult pieces?**