The expanding cosmos all began with the Big Bang 13.7 billion years ago. To discover what caused the Big Bang, we would need to know what preceded it. But that would involve the impossible task of estimating through the initial instant to when the entire universe was squashed down to a point of infinite density. Our physics cannot handle such situations.
And the problem actually goes deeper. The Big Bang saw the coming into existence of not only the contents of the universe, but also of space and time. Without any pre-existing time, there could not have been a cause of the Big Bang.
So it is not a case of us failing to answer the question ‘what caused the Big Bang?’ because the question itself is meaningless.