I think it's basically a matter of taste and I see a couple of reasons for the Scheveningen guy:
- he likes to put his knight on c6;
- he wants to develop Bf8 and castle faster rather than spend time with ...a6;
- he doesn't like the Sozin Attack against the Najdorf.
What are some reasons why one would play the Scheveningen rather than the Najdorf, given the Keres Attack and the flexibility of the Najdorf?