Well, if you think about how each set of organisms achieves it's locomotion, the answer starts to focus a little more clearly. Fish have more of a need for use of the pectoral fins to help them swim in water, while tetrapods (organisms who get around on all fours) have more of a need for the rear part of the locomotive mechanism to work.
This is probably another evolutionary mechanism that helps distinguish organisms from each other. When comparing structure of organisms, in an attempt to group them together into groups or families, it is important to do so according to existing similar physical characteristics. Therefore, given the striking difference between the two groups mentioned here, you would not expect to find them in the same general grouping based on this difference in locomotive apparatus.
There is an old saying, "Necessity is the mother of all invention". That probably could be applied here, as well. Fish had less of a need for hind quarters that worked as well as the front quarters, so over time, the hind quarters part diminished. Tetrapods had more of a need for a strong rear quadrant, especially when they reared up on their hind legs to reach the tastiest leaves in the trees they ate. So, I think it probably was need-based.