My theoretical speculation based on the past years is that they don't consider Allegri failed, on the contrary - they consider he's helping to keep the club alive with its head over the water. And they are thankful for that.
Results are probably considered decent, considering all the turmoil. This is a new board, that just assumed positions where there predecessors were nearly jailed. Football performance is probably the last thing they are worried about. They care about stability, experience, leadership and dealing with a guy they already know and can trust.
If the issue was the performance, he'd be gone a long time ago. The club is still recovering from another traumatic experience and as I said, I find more likely they buy more time into this process and renew him than actually try to overturn things and risk a perceived disaster, like with Pirlo.
Also, Allegri serves pretty well as a shield for the board, sharing responsibility from the public opinion. That should be an absolute priority for them.
Finally, you can tell by the transfer rumours/policy that Allegri is enjoying more power than ever at the club, because of the circunstances above. They really, really don't give a shit about what fans think. It's survival mode. Nothing indicates change, apart from a complete disaster this season. And they will absolutely NOT take into account the performance - only results. Anything close to decent results will do it.