- The more compensation amateurs were allowed to receive, the more the definition between "amateurs" and "professionals" became indistinguishable (if it wasn't already).
- Increased viewership.
Pierre de Coubertin, the founder of the International Olympic Committee (IOC) in 1894, had intended that Olympic competition should be among amateurs rather than professionals.
In the dictionary, "amateur" means:
a person who engages in a study, sport, or other activity for pleasure rather than for financial benefit or professional reasons.
However, in 1971, the IOC approved compensation for amateur athletes(1).
Eastern nations were sponsored to train and compete on a full-time basis by their governments. As a result, US athletes found it difficult to compete with athletes from eastern nations, resulting in the Ted Stevens Olympic and Amateur Sports Act in 1978 allowing US athletes to receive compensation.
Moreover, professionals had better brand-recognition than amateurs, as the following quote notes(2):
"The pros are there for a reason," the esteemed sports journalist Ron Rapoport, who has covered six Olympics ... "People will tune in to watch athletes they know. The pro athletes are pre-sold to the public, which means increased viewership."
After these developments, professionals became eligible for the Olympics(3):
After the 1988 Games, the IOC voted to declare all professionals eligible for the Olympics, subject to the approval of the international federations in charge of each sport.