Here's why:
* Homologous structures are structures that share a common ancestor. They may have different functions, but they share a similar underlying skeletal structure due to their common evolutionary origin. For example, the wing of a bat, the flipper of a whale, and the arm of a human are all homologous structures.
* Dolphin fins and fish tails have different evolutionary origins. Fish tails evolved from the tail fin of their aquatic ancestors, while dolphin fins evolved from the forelimbs of their terrestrial ancestors. Although both structures are used for swimming, their underlying skeletal structures are quite different.
* Analogous structures are structures that have similar functions but different evolutionary origins. Dolphin fins and fish tails are analogous structures because they both serve the purpose of swimming.
In summary, while both dolphin fins and fish tails are adapted for aquatic life, they evolved independently from different ancestral structures. This makes them analogous, not homologous.