Does anyone have have insight on what I'm doing wrong?
You're trying to 3D-track a shot that has no motion (in the critical parts, that is). There's not enough parallax to calculate the spatial correlations. This might be solvcable in a 3D tracking tools liek SynthEyes that allows manual placement of reference points/ trackers or even placing reference geometry and it might be solvable with mocha Pro even, but not in any of the "simple" tools that come with AE or The Foundry's Camera tracker plug-in for that matter... Unless you have access to such tools, you might in fact be quicker to just hand-animate the placement...
I would use Mocha to do that shot. You're trying to replace a plane and Mocha is a Planar Tracker.
AE's 3D camera tracker is not as well equipped to handle that shot. You might have more success if you split the shot and only try and track from the first frame where you need the image replacement, and that is what I'd do in Mocha. I ran a quick test. The key is to set your target near the end of the shot and make sure that you're using the right points in your track. See this screen shot:
I hand selected those points and then expanded the target to that it was large enough to give me a good indication of perspective match. You can then move the target so that it is centered in the area you want to replace.
I selected create camera and solid, added a grid to the solid to check alignment, and the shot worked quite well. You' would then simply replace the solid with your Photoshop file. Here's the important part. I cut the shot so that I was only tracking from the first frame where I needed the background replacement to the last. This will always improve your odds of getting an accurate solution.
Mocha, as I said before, would do a great job with this kind of a shot and it probably would have been my first choice.
I agree with Rick. I saw your question on another forum and gave essentially the same answer there.