The rules were changed to only allow seven men on the line of scrimmage and only one offensive player in motion because

it prevents all sleeper or hideout plays and  prevents a shift from occurring.

Explanation:

I inferred this question is referring to flag football game rules. Interestingly, flag football unlike other sports requires a team of seven players with a minimum of 5 to continue a game.

By allowing seven men on the line of scrimmage prevented hideout plays from unknown players, and since only one offensive player could be in motion it also prevented a shift from occurring.