It is important to note that John Steinbeck wrote Of Mice and Men with the intention of turning it into a stage play. According to the eNotes Introduction to the novella:
With the success of the novel, Steinbeck worked on a stage version with playwright George Kaufman, who directed the play. Of Mice and Men opened on Broadway in New York City on November 23, 1937, with Wallace Ford as George and Broderick Crawford as Lennie.
It can easily be seen that the book was written in such a way that the adaptation would be quick and easy. For instance, instead of describing the bus stopping and the driver telling George and Lennie to get off, George tells this to Lennie in the form of dialogue after they have reached their campsite by the river. Thus the theater audience would get the same information as the reader in the same way--through dialogue. Most of the exposition in the novella is handled as dialogue, and it is very convenient for Steinbeck that one of his characters is a retard, because that forces George to explain everything to Lennie and sometimes explain it all over again.
George gripes about the inconsiderate bus driver:
"Jes' a little stretch down the highway," he says. "Jes' a little stretch." God damn near four miles, that's what it was! Didn't wanta stop at the ranch gate, that's what. Too God damn lazy to pull up. Wonder he isn't too damn good to stop in Soledad at all."
Apparently the ranch property reaches all the way to the highway, but the ranch gate is about four miles off the main highway, and the driver didn't want to go out of his way for a couple of bindlestiffs.
This enables Steinbeck to set up his scene by the river, where the reader learns more about these two men, including the fact that they had to run away from some serious trouble in Weed. Here is an example of how exposition in conveyed in dialogue with the future adaptation in mind.
"Where we goin', George?"
The little man jerked down the brim of his hat and scowled over at Lennie. "So you forgot that aweady, did you? I gotta tell you again, do I?....You remember about us goin' into Murray and Ready's, and they give us work cards and bus tickets?"
In this fashion, all the necessary information is conveyed to the reader and the future theater audience. The play was produced in the same year the book was published, so Steinbeck was pressed for time. He also made the book remarkably short so that it could be adapted to a play that would show in a couple of hours.
Steinbeck had to explain why his two charactdrs were camping by the river instead of going straight to the ranch, where even Lennie understands that they could have gotten bunks and a meal. The author probably wanted to establish the setting where George would shoot Lennie in the final chapter. Shooting Lennie was a quick way of ending the story and making it short enough to fit into a stage play.
Steinbeck intentionally left out most outdoor scenes, including the one in which the driver tells them to get off the bus. This creates a little confusion as to the exact location of the ranch gate and the spot when they got off. Most of the story takes place in two indoor settings, the bunkhouse and the barn (which includes Crooks' room), with some sound effects suggesting horses stomping and men playing horseshoes. This is all done with the intention of making the novella into a play.