The primary complication is that objects move around between frames, so the list doesn't stay sorted. Insertion requires a second scan once I've found an object that is out of position, whereas bubble can swap right away. (The candidate object is always N-1, after all.)

My goal is to stop working as soon as possible; this is a 1.7 MHz CPU and it has many other tasks to perform on a given frame. It's hard to appreciate unless you sit down to actually do it, but on a processor this slow, the scanning of the list and the comparison of depth is not cheap. It's computed on the fly as we go, there isn't spare RAM to cache it, and it changes every frame anyway. All of that makes the scanning and comparisons way more expensive than the eventual swap.

Insertion would be ideal here for a freshly spawned object, since we already know it is out of position and merely need to find its destination. (The work to shift the rest of the list down isn't super expensive, thankfully.) In practice this is already a big enough visual change that the player doesn't have much time to notice a depth error before it corrects itself.