Balance of Numbers on dice
To prevent skill from being a factor in rolling

Dice have an even distribution of the numbers around the faces to prevent (or at least reduce) the ability to modify your chances of a result by skillfully rolling the die. Dice are intended to be entirely luck based, and depending on how the numbers are distributed around the faces of the die, could lead to skilled rolling (generally considered cheating).

It is for this reason that Spindowns are generally not allowed to be rolled anywhere that a random result is required (IE Tournaments, Organized Play, etc). For the same reason, d10s shouldn't be rolled to get an Even/Odd result as you can very easily force the desired result.