The true reason why Jay got the show.
The real reason was greed. At the time both 'The Tonight Show' and 'Late Night' were big money makers for NBC, still are. Back then those shows gave NBC a monopoly on Late Night. Nightline was the only real competition and it's only a half hour long, and it would only win when there's big news. NBC thought they could keep both shows and both guys. The prevailing thought was that nobody was going to go up against 'The Tonight Show' because everyone who did, lost. Fox had tried with Joan Rivers, Then with Alan Thicke, and Chevy Chase ABC was not going to interrupt 'Nightline' it's the crown jewel of the News department, and CBS which at the time have fallen in the ratings would never take a big gamble. But that was what CBS did.
NBC believed that 'The Tonight Show' was invincible. But Arsenio proved otherwise, although he never beat Johnny consistently he had his nights and Jay Leno was no Johnny Carson, CBS was counting on it and they were right, for the first 18 months Dave beat Jay. But then the Hugh Grant interview and the rest was Jay over Dave for almost 2 decades.
Badder than old King Kong, Meaner than a Junkyard dog