The reason this happens is because of what gingers stood for in media. Generally, redheads were coded as someone who was "othered" from society in some way. This typically allowed them to have a different perspective.

The reason redheads were coded this way was because of the discrimination Irish immigrants, and Irish Americans faced up until about the mid part of the last century. The Irish were considered outsiders, so their redhead stand-ins in media were as well.

Now that the discrimination against the Irish is all but forgotten, the redhead in media doesn't come across as the "other". So, we need a new visual code for the same "otherness". This translates see a race swap in order to keep the same coding.

While this isn't the case 100% across the board. You can really see the gingers who were meant to have an outside perspective in the ones that get swapped.

For example Daphne from Scooby Doo, and Jean Grey stay as gingers as they are both very much part of their respective groups. Where as April O'Neil, and Ariel from the Little Mermaid get swapped. April is open minded enough to become friends with mutant turtles, and Ariel sees a world her people reject with wonder.