His family didn't approve of her
Diana's former status as a royal princess still wasn't good enough to win over the Khan family's approval. You see, Khan was a Pathan, a group of people between Pakistan and Afghanistan notable for their fierce loyalty to cultural traditions. After two failed attempts to marry Khan off to a suitable Muslim bride, they began to grow impatient. But the affluent Khans weren't going to give up on their oldest child that easily! In an exclusive interview with the Daily Express, Khan's father, Dr. Rashid Khan, made it very clear that Diana had no chance of ever being a suitable bride for their son. "He is not going to marry her," he said. "We are looking for a bride for him. She should be rich, among the upper middle class. She should at least be a Pakistani Muslim girl." If you were shocked to learn about Diana's secret love affair, you won't believe these 10 fascinating facts you never knew about Princess Diana.
11 Secrets You Never Knew About Princess Diana's Love Affair, Source:https://www.rd.com/list/secrets-about-princess-dianas-affair-hasnat-khan/