Because, after many centuries of common law in England, that’s the number that’s been found to give reasonably fair results in most cases. But that’s begging the question ! Why 12?
Because that’s the average number of mature adults in a typical hunter-gatherer tribe. When this number comes to more 12 — that is, when the tribe is around 120 total individuals or more — the dissention inevitable occurs and the tribes invariably splits into two around an existing leader and a forceful potential new one.
A group of about five or six individuals is probably the maximum size group where a totally unified decision can be quickly arrived it. Irrespective of whether all such decisions are good or bad, this is roughly the size of group that can takes firm decisions as in boards of directors of large firms or government cabinets.
Back to juries, a group of 12 is probably about the best number in which a dissenting voice is encouraged to speak up and gather supporters if the verdict was too easily going one way too soon — but yet not cause deep problems.