First off, Finnick Odair is one of the best swimmers in his district.
Which works perfectly because Nathan is one of the best swimmers in the world.
In The Hunger Games, Katniss attributes Finnick's success to a combination of his extraordinary physical skill and his unprecedented popularity among the Capitol.
Nathan is both popular and extremely skilled.
Katniss also says no other could tribute could overcome Finnick's "natural beauty".
Nathan Adrian has a lot of natural beauty.
Though, like Finnick, he does have his moments.
Nathan Adrian also doesn't really wear pants.
They're also basically the same age.
And last but not least, he likes sweets.
H/t to ONTD. for making me realize this.