Remember that 1. It took the introduction of mandatory public schooling and the banning of any language other than English to get the Cajun and Creole populations of Louisiana to start speaking English. In other word, it took government action to correct their behavior. As a matter of fact, little cajun kids would have their noses rubbed into the chalkboard if they spoke French. Would you like to do the same for the kids of PR?
2. The areas aquired from Mexico were largely uninhabited, with cities such as Los Angeles and San Francisco, to say nothing of East Texas, already largely Anglo by the time of the Mexican War. In those regions where the Latinos remained dominant (western New Mexico, South Texas) Spanish remained the language of choice, although, like Lousiana, the introduction of mandatory public schooling in English caused its usage to dissipate by the mid 1950s. It has only been in recent decades with increased immigration from Mexico that Spanish has made a comeback in the region.