These recordings are magnificent - even the blemishes just bring it all to life, and it is hard to believe that these were recorded in the 1930s (Casals once played, as a young man, for Queen Victoria).
The tempo, the earthiness of the timbre of the instrument, and most of all the pure feeling that Casals brings to this work are what sets it above other versions (some of which are too fast, some are too pure or engineered, and all of which sound more mechanical). It's a combination of a special piece of music with a special artist delivering a rounded and, somehow, complete interpretation of this work to the the listener.
In response to the other reviewer - according to the sleeve notes of the version I have I do believe in fact that Casals did somehow "discover" these pieces and so he really owns this work, in a way. And when you have an unaccompanied instrument like the cello (which is not spatially large, and which delivers a fairly narrow range of frequencies), whether you stick one, two or many microphones around the player doesn't make much difference, and this is why the fact that it is in mono does not detract from the work at all.
If you like this, check out Glenn Gould's 1955 version of the Goldberg Variations by the same composer - equally magnificent for the same reasons.