It's because Crooners talked about more then
>A)Cars or other expensive items
>B)hoe/bitches
>C)Money and the making of
>D) A combination of the above.
Granted that's a generalization, but that's what people think of for shitty rap.
Also it's more about rhyming not having rhythm (Most music has A Rhythm, and you have to get into expermental stuff for it not to have one). The reason why Rhyming is impressive is because it's niggers managing to find two words that sound similar (and some time they still fail to do it).
Crooners were more about harmonizing and had range.
Not saying Crooners the best thing ever and there's no place for Rap. But they're different era's that focus on different talents.