I would say its because no one really decided to change anything. No one during the old ages really knew how to do anything and only used what they had and what was closest to them.
The Mexican American war was basically when The U.S. Invaded mexico during the period April 25, 1846 – February 2, 1848:
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The dispute grew up from the United States’ annexation of </span>Texas<span> in 1845 and from a dispute over whether Texas ended at the Nueces River (Mexican claim) or the </span><span>Rio Grande</span>
California, New York, Texas, Tennessee and New Mexico
