To add on to that the complexity of developing such weapons grows considerably when you take into account the uniqueness of each target - various resistances, adrenaline, different body composition, hit area and so on. If you fail to incapacitate the target they might retaliate lethally.
Which is actually possible with lethal weapons too - people don't always die from a shot or two and not always instantly. So to ensure the threat is defused the weapons must be very effective and the best way to ensure that is by making them as lethal as possible.
Indeed. And on the other side of the equation, if you overdo your non-lethal weapons, they can easily become lethal. Tasers have already claimed many lives. That ball of fast-setting cementy thing you shoot at your target may solidify in a way that chokes your enemy to death. And this is all discounting the possibility of a disabled enemy falling to his death by accident.
Effective non-lethal weapon is a very narrow target to aim for.
Which is actually possible with lethal weapons too - people don't always die from a shot or two and not always instantly. So to ensure the threat is defused the weapons must be very effective and the best way to ensure that is by making them as lethal as possible.