Why did Doc disappear from Cars?

The reason Doc Hudson disappeared from the Cars movie series (it was established in the story that he had passed away) is that the legendary Hollywood actor Paul Newman, who provided the original English voice, passed away in 2008.
Pixar chose to pay tribute to him and did not cast a replacement voice actor. Consequently, the story was changed in Cars 2 to reflect that Doc Hudson had also passed away.
However, in the third film, Cars 3, Doc Hudson was brought back in a flashback scene by using previously unreleased audio recordings of Paul Newman that had been captured on a microphone left running during the recording sessions for the first film.