Why is the U.S. not getting more of its oil from its own supplies or Mexico?

The most important reason is the extensive demand. Current US' domestic supplies provide approximately 60% of the country's needs, while 40% (some 11 million barrels per day) have to be imported from other countries. While current technological advances are allowing lesser imports -- i.e: expansion of renewable sources or extraction on more difficult places, such as the Arctic Circle -- it is very difficult to be completely energy-independent.

On the other hand, most oil is imported from Canada (28%), Saudi Arabia (13%) and Mexico (10%). Nowadays, Mexico has reached a "peak oil", meaning its capacity for oil extraction has stagnated, even diminished due to dwindling reserves. Mexico is currently trying to alleviate this problem by allowing foreign companies to invest in oil extraction on Mexican soil and waters for the first time in over 70 years, but at least for the time being, it cannot increase its production.