Because they are Kate and Gerry.
Publicity loving (unless it's negative), media savvy, they always assume everyone wants to hear their opinion. Blogs and books and interviews confirm that.
Indeed, the British media has traditionally given them a platform to air their displeasures openly, and a sympathetic slant to even their most narcissistic displays.
Like this one.
But the point is Why? Who is advising the McCanns that they openly go against SY when this clearly
diminishes their standing rather than the opposite? We would not be sitting here discussing this issue in negative terms if the McCanns had decided it was wiser to present a united front vis-a-vis SY, whether or not they saw eye to eye. Wouldn't the benefits of that outweigh the merits of making their own individual points, however important they believed them to be, thus displaying a rift?
There is the famous adage of the person approaching a green light, who sees a car coming fast towards him from a side road. He has right of way on the green light and is at perfect liberty to sail though the light and ignore the car coming from the side. If he does not break, however, the car coming from the side will smash into him.
There is often a conflict between what is strictly speaking right or what one has the right to do, and what is the sensible course of action to take in the circumstances. This seems to me to be another example of the McCanns exercising their right to do things their way. But at what cost to their relationship with SY and the success of the investigation in general?