It was a decision of the director José Padilha, who cooperated with Moura in the past:
A Brazilian former journalist, Moura made his name starring in the
Brazilian police thriller Elite Squad and its sequel, that were
directed by Narcos producer José Padilha.
What drew you to the role of Pablo Escobar?
The first thing was the relationship I had with the director of the
show José Padilha —we had worked to together on his to previous films,
and we established a very strong collaboration and relationship. So
when he was invited to do Narcos, he just decided I was his choice for
Regarding the accent, I believe that Padilha was aware that Moura's Escobar won't sound genuine, but as the show was targeted at international audience he possibly preferred the actor's quality over the authenticity of the accent:
But Padilha knew the tender and thoughtful Moura could transform
himself. He had directed Moura in 2007's Elite Squad and its sequel,
Elite Squad: The Enemy Within, the highest-grossing Brazilian film
ever. To play a military police captain, Moura submitted to boot camp
with Rio's elite police. "Many actors gave up and said, 'Fuck it, I'm
an actor. I don't have to be here,' " Moura says. He stuck it out.
In addition to the above, the show was initially planned to be shot entirely in English. Three months before the shooting started the plans changed and it was decided that Spanish language dialogues will be shot in Spanish. This could be one of the reasons why not only Escobar, but also some other main characters are played by non-Colombian actors:
Colombian audiences have been irritated and amused by the show’s
hodgepodge of accents. A character based on Escobar’s wife is played
by Paulina Gaitán (Mexican), his partner Gonzalo Rodríguez Gacha by
Luís Guzmán (Puerto Rican), while his rival Jorge Ochoa is played by
André Mattos (Brazilian).