Merckx reputedly didn't win the
Tour more often because at one point the direction asked him to stay away - he was spoiling the event.
Eventually it turned out he wasn't invincible - the last Tour he participated in was a failure. His body had run out.
It's also worth mentioning that doping was important, probably crucial, to winning cycling races in those days. So was politics: team play. What I found most enlightening in this respect was a discussion on Belgian TV between Merckx and two other champs of the sixties, Rik van Looy and Rik van Steenbergen. They were openly saying things like "well the reason your man didn't attempt to catch me on that day was that I'd bribed him". Winning races was a matter of making the right deals, and Merckx clearly knew all about it. But his nickname 'the cannibal' stems from the fact that most of the time, he ignored all deal-making and simply raced a lot faster than anybody else to win the event. This is why he is considered the greatest of all time.