Because it's such a tough sport. The Tour de France was originally intended to be so tough that only one person might finish it. In other words it was set up to be extremely hard for most normal athletes to compete without some kind of artificial assistance.
So there was a history of drug taking from the start. But after the scandals of 20 years ago it became one of the most tested sports in the world. So now, in my opinion, drugs are not used much compared to other relatively untested sports (maybe some microdosing). Instead sports science has taken over. Pogacar, the current TdF champion works with a someone who is a contributor in mitochondria research. Something that has made a big difference in the last few years is the amount of carbohydrates the riders take in during a stage etc. etc.
So there was a history of drug taking from the start. But after the scandals of 20 years ago it became one of the most tested sports in the world. So now, in my opinion, drugs are not used much compared to other relatively untested sports (maybe some microdosing). Instead sports science has taken over. Pogacar, the current TdF champion works with a someone who is a contributor in mitochondria research. Something that has made a big difference in the last few years is the amount of carbohydrates the riders take in during a stage etc. etc.