I heard a broadcast interview once where Brendel was asked why he did not play certain composers' works. He replied that there were some whose music he did not much like. I think Rachmaninov was one of those. Then there were others whose music he loved, but to do them justice would require such specialisation that he would never have time to study them properly. Chopin was one of those.
In short, he was a perfectionist in many ways and would not perform anything that fell short of his high standards. Personally I do not like everything he has recorded, for example he has (IMO) a habit of pushing Mozart and Schubert beyond their limits, but he is a great interpreter of Beethoven (especially the late sonatas).