Why were immigrants from Mexico not included in the quota system set by acts?

Mexicans did not fall under any national-origins immigration quotas set in such laws as the Immigration Act of 1924 (Johnson-Reed Act) because Mexico (plus most countries in the Western Hemisphere) was exempted from this immigration quota system. Under Johnson-Reed and its succeeding amendments throughout much of the 20th cent., citizens within the Eastern Hemisphere were assigned percentages based on a formula in relation to the numbers already residing in the U.S.; while Western Hemisphere and Asian immigrants generally entered without such restrictions.[