Nashville is about $$$.
If suddenly a country artist started having tons of Black ppl in their concerts, buying their albums, you can bet they would find another in the same vein.
They didn't care that Taylor Swift wasn't traditional country, b/c she was brining fans in by the boatload.
Darius Rucker is a great artist w/ a great voice who already had a name for himself. And some of those Hootie & the Blowfish songs could pass for country music in 2015. So white people of all ages were already familar with him.
And truth be told, country songs are the last remaining well written/crafted songs in popular music. They sold out the traditional style for more fans, but they didn't sell all the way out like R&B. That's why it's still a hot, self-contained genre.