The events of Narcos, which are based on a true story, took place mostly in Colombia where the official language is Spanish. It made more sense to have the show contain Spanish dialogues.
Since many of the significant characters in the show are Colombian, it made more sense to Netflix to make the show in Spanish in order to make it seem more realistic.
According to Future, Netflix might have made the show in Spanish to reach the broad market of Spanish speaking Americans who are believed to be more than 55 million.
Some people theorized that Netflix might have made such a move in order to grow faster in Spanish speaking countries. See how Netflix started.
Netflix might have adopted this strategy to make people embrace shows that have different languages. As people get used to subtitles, Netflix could introduce more content from all around the world.
Spanish is the second most spoken language in the world. Netflix could have made this move to increase its subscribers and to grow its business.
During the show, Netflix sometimes introduces real scenes from the real events that happened. The fact that many of the real scenes include Spanish speakers might have made it more logical for Netflix to make the show in Spanish.