Partially because there hasn't been user demand for it to date. Such a change will require the programming community to start clamoring for tools to make things secure. We want speed, easy to use syntax, things like that.
You'd be hard pushed to find a framework or language that didn't have, and recommend, ways to access a database that guarantee injection can't happen. If people choose not to use them, there's not a whole lot you can do.
That's just a small part of the puzzle, though, and not every security issue would or could be fixed by solutions of that nature. There is no fix, other than developers being informed, capable, and diligent.
