So are you asking why people believe this or why Hispanics actually seem to place less emphasis on education?
I would say that people believe Hispanics place less emphasis on education because they end up being less educated than other ethnic groups. For example, the link below says that:
Hispanics have a much higher high school drop-out rate than do blacks or whites. Some 41% of Hispanic adults age 20 and older in the United States do not have a regular high school diploma, compared with 23% of black adults and 14% of white adults.
This sort of statistic might tend to make people think that Hispanics place less value on education.
If you are asking why Hispanics have such bad numbers in this area, there are a variety of possible factors. One would be the language barrier. Blacks and whites, even if poor, do not typically speak a language other than English at home. This makes education easier for them. It may also be that many Hispanics end up having to drop out to work. There are other possible factors, but these are two of the more likely causes of low Hispanic educational achievement.
Actually, it is both but mainly Hispanic immigrants here in U.S.