II believe the answer is Ivan Ilyich cannot see that Praskovya Fedorovna genuinely cares about his health because he hates her.
In the story, Ivan Married praskovya not because of love. So, he really hates his wife and often treat her badly and making Praskovya's life miserable no matter what Good behavior Pra. But, Praskovya actually really care about Ivan and not wanted him to suffer because of his illness.