I'm not sure why WordPress did this. But the ability to change a post author used to be on the post screen by default. But now it's not.
Here's how to make it visible again.
https://skitch.com/mclanecreative/f451m/turn-on-authors
1. From the post screen, click on "Screen options"
2. Check the author box
3. Do a little dance
Head of the monster tribe. Lead family gardener.