Blame Cy Williams.
Williams was the National League's career home run leader through 1929, before ceding the top spot to Rogers Hornsby. Opposing managers would position two outfielders in deep right in an effort to catch anything that Williams didn't hit over the shallow right field wall in the Baker Bowl.
Or maybe the fault belongs to Ted Williams. In 1941, White Sox manager Jimmy Dykes instructed his second baseman to play in shallow right field, directing his shortstop to move to the right side of the keystone while positioning his third baseman where the shortstop usually plays. Sound familiar? The famous "Williams shift" is the precursor to many of the defenses currently deployed by MLB managers.