To give it a more positive spin: Turing's case highlights how not only unjust but simply stupid such prejudice is. His case provides a very public and undeniable example that keeps the issue in people's minds, essentially a form of martyrdom, highlighting how far we've come and (when you consider how much prejudice and related bad thinking is still common) how far we still need to go before we can truly consider ourselves civilised as a whole.

Also his brilliance does come into it once you consider the wider implications. What else could he have achieved (both for his own intellectual satisfaction and our eventual benefit) had he been allowed to continue his work as he wished? How many other people are out there (or were out there) who could have done great things but were not permitted to by society? While I agree that is is bad that we need him to stand as an example to highlight these issues, it is good that he does stand as that example to shine that light on the problem.

It's mostly because human beings feel more sympathy for a single victim who they know a lot about. I think the term is "Identifiable Victim Effect".