Because his writing was of ideas as well as of people, and therefore gratifies the intellect. He belongs to a special group of writers who makes you re-examine many things you take for granted, by assuming a small variation on reality, and taking it to its logical conclusion.
He was really a language hacker. Most people looked at his writing as most programmers look at Lisp. Ignoring some syntax facilities and abusing on others he could convey meaning as nobody else.
While most writers use syntax conventions to communicate as clearly as possible, Saramago adulterated syntax to communicate on a different level.
At first it's frustrating, and his lack of respect for most of the conventions or beliefs may throw you off, but then you understand he mastered writing expression as few to transmit his thoughts.
His writing is a landmark for me as an example of Portuguese language use. While I envy and admire English simplicity, Saramago's work remembers me why it is a privilege to be born as a native Portuguese speaker.
I also would like to speak/write better English, no disrespect intended.
I don't think so. He is a great writer. I use HN as my home page and if I did not know about Saramago this post would have introduced me to his writing and I would have been grateful for this.