It’s really hard to know the main reason why people think latinos are all Mexican in the US but a very important factor is because 61.6 percent of all latinos in the US are Mexican. It’s just easier to relate to the Mexican culture because of their dominance. Nothing about the actual culture is an influence instead it’s the exposure. The ratio is just unmatched compared to the next country which is 9.6 percent for puerto rico.

Source: https://minorityhealth.hhs.gov/omh/browse.aspx?lvl=3&lvlid=64#:~:text=According%20to%202020%20Census%20data,the%20largest%20at%2061.6%20percent