I can tell you why right now. I've been waiting for these things to show up in stores for a year and they're just not. Nobody is stocking them near me, which is super disappointing because I really wanted to try out both models and see if I preferred one over the other. Even now, there aren't a lot of youtube videos of either model. I have to wonder if they're selling. I'm still concerned about the neck being wider than the Jazz necks I'm used to, which is why I'm looking to order from someplace with a good return policy.
At the end of the day I'm a classic hard rock player, and most of the classic bass tones I like come from Rickenbackers or Fender Jazz basses. Not many people know this, but the Jazz bass was actually created specifically to compete with the Rickenbacker 4001. The Kestrel, with its neck-through construction and single-coil pickups, really seems like a combination of those two basses.
I like an agressive midrange and top end to my bass tone and from what I've been able to hear from the demos, the Kestrel will probably deliver on that better than the Kingfisher.
The other reason is ergonomic. The Kingfisher's neck is longer on account of the two extra frets, and the bridge is moved up. This is an effect I love on my Custom 24, but I suspect the Kestrel will be easier to play. Again, I'd have preferred to try both models out in person, but it doesn't look like that's happening any time soon.