Why are london's short stories considred his most valuable contribution to literature?

Answer 1

London's whole thing is to put someone into one hyper-tense, very difficult and dramatic situation and use the character to ask the reader "What would you do?" And, since he's London, he's also thinking "And why haven't you gotten yourself into this position yet?"
This works better in a short story, because there is basically one idea (not a plot, per se) and the purpose is pedagogical. The Sea Wolf is too long to be built on such a situation and it falls apart. The London situation can be a climax of a novel, but it is very difficult to make it the entire raison d'etre of a novel. So, it works really well in "To Build a Fire."
Also, most of the people who really like London are fairly young. Short stories are easier for a child or busy student to read than a novel.

in short, his other contributions really weren't that valuable