Why do whales have pelvic bones and fish don't?

Whales are mammals ; see relevant link for additional information .

Whales have pelvic bones because millions of years ago, they were land animals, but as they evolved, they no longer needed their pelvis because over time they lost their need for legs. So now their pelvis and legs are vestigial, meaning they no longer serve their original purpose. They just kept the trait. As for fish, they never had legs, so no need for a pelvis.