Because it may become one if without proof the PM is sanctioned by the courts on the basis for what someone assumed he might be doing.
It would set a precedent for actions taken by a future PM's where, not only in terms of possible proroguing, but anywhere their actions are open to interpretation, but lack proof, on matters normally within their gift/remit.
If Johnson admits he misled HM (probably have to wait for his memoirs for that), or if, as someone else said earlier, an email from Johnson himself comes to the fore indicating his reasons were not what he stated, then the courts have a role, but otherwise they do not, as I am sure the Supreme Court will rule.