Question: Why did Gene Hackman quit acting after appearing in this movie?
Answer: As the other fine answers indicated, it was multiple reasons. An additional one was his health. Hackman said his doctor told him his heart wasn't healthy enough to withstand the stress and physical rigors involved in filming movies.
Answer: It was said, that he got tired of doing movies where everything and everyone explodes and dies a bloody death.
Answer: His decision to retire wasn't related to this film; it just happened to be his last. He was working on other projects (mostly writing), and wanted to focus on those. He didn't "officially" announce his retirement from acting until 2008.