Leno got the Tonight Show for the same reason that NBC is dumping serious programming to give him the 10 slot. Money.
Back in 92, NBC had the choice of paying Letterman fair value, in which case it would have dominated late night as it had with Carson, with no serious competiton. Or taking Leno, knowing the Letterman would go elsewhere and split the audience. But it got Leno cheap. It went for money over quality.
Now NBC is giving up on the 10 slot. Again it understands this will not be a high rated show, but a Leno talk show is far cheaper than five filmed dramas.
I do agree that Letterman has ROAD for the last few years. Perhaps its a measure of his age. His frat boy humor just does not work for a man in his 60s, and his foray into political humor on an unfair basis (unlike the "old" Letterman or Carson, who were always even handed), just tarnishes his legacy.