Fair question. Orr was a defenseman so he didn't have the kind of gaudy stats that Gretzky and Lemieux did. Lemieux's career was cut short due to injuries/illness. He was legitimately on pace to be in Gretzky territory but he missed an entire season due to cancer treatment and retired early due to health issues and injuries and then came back for 5 seasons and retired again due to an irregular heartbeat. There is always the question if he would have become the greatest of all time if he wasn't plagued with so many issues. For some context on how great he was. The year after he came back from cancer treatment he had 161 points, led the league in scoring and won the MVP.

Might be some era bias on my part as well since I never saw Bobby Orr play but some people think he was the best ever because he was so great as a defenseman.