I can think of two reasons, 1) demographics and 2) culture.
First is sheer numbers. The number of Hispanic-Americans has risen from 10 million in 1970 to 50 million in 2010. In percentage terms, that's a rise from 5% of the U.S. population, to 17% over those 40 years.More to the point, it took that long for a generation of Spanish-speaking children to grow up and "take over" the music industry. A ratio of 38% Spanish songs to 17% (more like 19% today) is "only" 2 to 1.
The other question, of culture, is "why is the above ratio 2 to 1 and not 1 to 1 or less?" The reason is the greater push for cultural diversity. Over the past 50 years, American society has gone from "three flavors" (chocolate, vanilla, and strawberry) to Baskin-Robbins 31 flavors. More to the point, America is now exhibiting a preference for "spicy" (e.g. Hispanic) "flavors," over formerly bland Anglo-Saxon ones.