Why does Jack London use Siwash words?

Depends upon what you mean by "Siwash". Some assume the term identifies a specific group or tribe of Native Americans, but that is mistaken. The Word itself was originally Chinook jargon (trading language) for Native Americans in the Pacific Northwest, derived from the French sauvage (or "savage"). The term was used interchangeably with "Indian". The Seattle Siwashes were a PCL Baseball team (before they were named the Indians, then the Rainiers), which shows how ubiquitous the term once was, at least in the PNW. It fell into disuse because of its racist overtones, to the degree that most, unless they've read London, arr unlikely to have ever come across it.

So, there are no such things as "Siwash Words," in London or elsewhere. Certainly, he does not hesitate to use the jargon of the times and locales he is seeking to depict.