Battery life alone isn't the main point!
Extreme battery life + extreme performance is the main point!
The Thinkpads with extra battery weren't in the same "small and light" class as the Air and MBP 13. These "small and light" laptops with 20 hour video playback are faster than current workstation class laptops!
I think it is more about the fact that apple successfully makes things "mainstream". For instance, the air form factor had been done before by Sony (and those even had hires screens), but only got popular after the air.