"They thought they'd be able to come to our wedding but, in fact, they haven't/weren't/can't."
This is a sentence from an exercise on ellipsis and the right answer is "can't". What is wrong with "weren't"? Can't it be interpreted like that, the wedding was some time ago and they weren't able to come. So aren't there two correct answers, depending on whether the wedding was in the past or is going to take place on the future?