Let A be the best valid argument that has been given for an open future. But I have really excellent reasons to think that it's true that my ears won't turn to diamond over the next hour, reasons clearly stronger than my reasons to think that all of A's premises are true. But if there is an open future, then it's not true that my ears won't turn into diamonds over the next hour (since that depends on indeterministic quantum phenomena). So I have on balance reason to think A is unsound.
Of course, there are other arguments for open theism. But I can pair each one with a fact about the future that I have reason to be a lot more confident in than the truth of the argument's premises.