Why is it significant that Of Mice and Men was intended to be adapted into a play?

It is easier to understand the novel Of Mice and Men if the reader keeps in mind that Steinbeck called it a "playable novel" and was writing the story with the intention of turning it immediately into a stage play to be produced in New York in 1937, the same year the book was published.
Because the story had to be translated directly into the play, Steinbeck was forced to make it short, which explains the quick and final ending when George shoots Lennie--and the story is over. It explains why there are only two or three sets, and why these sets are so simple. Most of the action takes place in the bunkhouse, which can be represented on the stage with a few bunks, a wood-burning stove, and a big table where the men sit around on upturned wooden boxes and play cards. The other main set is the barn, which can also be simply represented. Sounds are heard offstage in both the book and the play. We hear horses stomping and harnesses jingling. We hear the clang of horseshoes and a dinner-bell ringing. As in a play, the exposition is conveyed mostly through dialogue. (Steinbeck was one of the best dialogue writers of his time.) The fact that Lennie's memory is poor provides the author with a good excuse for having George explain the past, present, and future to his friend in detail--and sometimes explain it all over again. Since this is a story about men who tramp the roads, ride on freight trains, and work in big, open fields, one might expect scenes set in the great outdoors. But these are conspicuously missing in the book because they could not be reproduced on a stage. Both the book and the play were very successful. They came out at the height of the Great Depression, a time when there was strong public interest in the plight of the working man and in radical measures to correct social ills.