I am not sure if this is true for just wrestlers. I was in Fight Sports Miami where I went against a white belt who was an ex division 1 defensive ene and he was very difficult to control. Not impossible though because I got there. Because athletes are better than your average joe who does this as a hobby.
So here is my take on it. A wrestler who had wrestled their whole life is probably a better athlete than the average BJJ practitioner. I never wrestled but if they went through the high school system and made it to the college system. They are probably pretty skilled and a better athlete than someone who did not do combat sports from the first half of their life.
You would be surprised how far great athletics prowess can take you. So now you take that person and teach them a sport that is not too different from their combat sport from a practical point of view. The purpose of both sports is to take the opponent to the ground and dominate. BJJ does it differently because submission is more important and giving up your bottom is not a big deal. Wrestling is more about controlling your opponent on the ground and pinning them.
A wrestler has probably 10+ years of combat sports experience before they enter BJJ. The average BJJ guy is probably a hobbyist. The wrestler has lots of competition experience probably more in 2 years than a BJJ guy in 5 years. The wrestler has suffered grueling practices and weight cuts. And the average BJJ class is not that tough. So it is that unexpected to believe that a competitive athlete can be better than the average person doing BJJ? Especially when some ofmost important aspects of jiu-jitsu is taking your opponent to the ground and control. A wrestler has a lot of experience doing that.
So that is why they have an easier transition. I mean look at Nicky Rod got an ADCC silver medal primarily using his wrestling. Why because taking your opponent down and controlling them is important in grappling. And if we take belts out the equation you wouldn't call Nicky Rod a beginner in the grappling arts. He may have been a beginner in BJJ at the time but not a beginner in grappling arts.