If your answer is "it's a random mutation" then that settles the "why" question permanently. Why all this idle speculation about bee's stingers, then? It was a random mutation, and it survived, done.
Every single part of an organism goes through a recombination/mutation process countless times, the stinger evolved to be what it is today over a very long time and it's cool to study why it ended up the way it did. Tells us about their environment and history and evolutionary pressures, survival is a result of the random traits being successful in their context in specific ways.
Why shouldn't different or even "opposite" traits also be successful? When faced with random inheritable differences across different species over long periods of time why wouldn't the result be a variety of them, every one of which just didn't prevent reproduction from passing those traits on to the next generation? Some traits might be seen as "better" or "worse" by comparison but as long as they get passed on, we'll see both. It isn't about being "best". It's about being "good enough"
Because the trait is not the end all be all. It's a random walk to an outcome that leads to enough offspring to survive for the species to be there at all. It's probably not even the most optimal solution whatever is there, just happens to be competitive among the rest of the ecology to not be snuffed out.
