For the same reason JKR wanted to put Hermione with Ron: she likes funny men. And redheads.
Since no one at the Yule Ball in 1994 ended up with the person they took than as a date, I also believe Fred wasn't meant to end up with Angelina.
And there's the fact that at some point JKR considered killing Ron ( theguardian.com/childrens-books-site/2011/oct/31/jk-rowling-ron-weasley )
Since JKR made Fred Weasley die for no other obvious reason than because the entire Weasley clan could not be saved from the wizarding war, and since he's the funniest of the set of twins, I place my bets on him for Hermione's alternative soul mate. ( today.com/popculture/rowling-i-wanted-kill-parents-2D80555846 )