Mostly because Syndra is one of the strongest mid laners right now, and is a hard matchup for Ahri. So picked Ahri with Syndra open is too risky.
I think Ahri also isn't particularly strong against Ryze or A.Sol, but I could be wrong there. I personally like the A.Sol matchup but I don't think it's supposed to be heavily favored for Ahri.
Then there's the big problem that Ahri does a lot of things well, but nothing great. Her damage is too low to solo carry if she gets fed. Her CC is too low to be a huge utility if behind. Her AOE is too low to wreck teamfights in closed quarters. She's a jack-of-all-trades yet master-of-none, so she doesn't fit with many specific worlds comps.