I heard one of his 'country rap' songs come on the other day
That is why Nelly fell off
He had his lane, he should've stuck to that lane, he should've collaborated with some other people that were close to that lane but he decided to go 'whiter than mainstream' and once an artist says "Ayo, how you know me is not really all I am" two things can happen: they can become HUGE (The Weeknd) or they can die off hard (Nelly). Also it's about time, some artists have a certain shelf life for inexplicable reasons but competition, a weird change (country??) can play a role.
Also: he was very private. He dated Ashanti, but dating Ashanti 5 years after she's on top is different than being e.g. Justin Bieber dating Hailey Baldwin or Tyga dating Kylie Jenner (probably without Chris Brown and Kylie Jenner Tyga would've just been some dude with a 1-year shelf life).