Pro cyclists are paid to ride races, which they do very frequently in the cycling season. The better the heart of the young pro rider has developed and adjusted to the extreme efforts of cycling the lower heart rate will remain in races. A low heart rate during races means that the intensity of the race is sustained well. In that case the rider recovers faster and he will be fresh for the next race.
Look at the heart rate curves in the article: The intensity of cycling races. One of them is the curve of an experienced pro rider with a well developed heart who bears race intensity very well.

A young rider who switches to the ranks of the pros with an insufficiently developed heart will be overburdened by the large number of intensive races which come with short intervals. This is expressed by the heart rate registrations which are constantly in the red zone. For this new pro the intensity in the races of professional riders is too high. The large number of races that succeed each other with short intervals renders it impossible to recover or recuperate completely. With overburdening and overtraining as a result. With the switch to the ranks of the pro riders the young cyclist should be sufficiently developed to deal with the rapid succession of intensive races. Only in that case he will succeed as a pro and develop further.

Young cyclists who wish to become professional cyclists should be aware of the necessity of sufficient heart development.

Developing a strong heart is a condition for a young cyclist to succeed in the professional category.
Pino Cerami
Wout Poels

Pino Cerami celebrated his greatest successes as a professional cyclist at older age. He was well over 30 when he achieved his greatest victories.

Wout Poels was considered a great at the start of his professional career. His first years as a professional rider he performed very changeable.. But after this period, he became an excellent climber and the main helper of Chris Froome in the mountain stages during the Tour de France of 2015.