I also thought this would be about Selena Gomez
Anyway you are completely right Selena Quintanilla is more of a Mexican-American icon that's where she was most successful, but there she became an icon. Her fanbase was just that strong. Also her music is pretty catchy and has a lot of replay value.
The reason why she's cited as being that big is because Mexicans in the US are the larger ethnic group so what they like becomes often the most popular thing among Hispanics in general in the US (this goes for TV shows as well).