Perhaps we’ll never know (https://en.wikipedia.org/wiki/Alec_Guinness):

However, Guinness soon became unhappy with being identified with the part, and expressed dismay at the fan following that the Star Wars trilogy attracted. In the DVD commentary of the original Star Wars, Lucas says that Guinness was not happy with the script rewrite in which Obi-Wan is killed. However, Guinness said in a 1999 interview that it was actually his idea to kill off Obi-Wan, persuading Lucas that it would make him a stronger character, and that Lucas agreed to the idea. Guinness stated in the interview, “What I didn’t tell Lucas was that I just couldn’t go on speaking those bloody awful, banal lines. I’d had enough of the mumbo jumbo.” He went on to say that he “shrivelled up” every time Star Wars was mentioned to him.