This actually answers everything I was wondering, thank you very much. So if I was hypothetically married to the witch knight set would Vipers sting make the weapon damage proc constant?
Is the question about pairing them together, or are you asking why those sets are not considered popular?
I'm certain you could make a fun build with them for sure. I'm not an expert on min-maxing sets, but here are some considerations.
Both sets' 5-piece bonuses are conditional on outside factors.
+Crit chance is considered the strongest damage property in PvE. Witch-Knight gives zero.
It's a good chunk of +Weapon damage but conditioned on you having poisoned or given a poisoned effect somehow to the target. Likely you'll easily achieve this by using poison skills but it's limiting your overall skill selection in a way by requiring you to make the enemy poisoned/poison DOT'd or the 5-piece doesn't do anything.
So without first poisoning / DOTing targets, you can't benefit from Witch Knight on say a Hail of Arrows -- unless your bow poison enchant or alchemy poison happens to proc.
Widowmaker is RNG based, and requires you to be using an alchemy poison on your weapon(s), and additionally for that poison to proc. Something like 20% chance I think?
So it definitely isn't a guaranteed thing to get that poison ball in short fights.
Further, the poison ball is location bound, it doesn't chase after or stay on the targets. If your target died or moved, and nothing else is standing in the poison -- there may as well not be any poison ball. 5-meters is a decent range but mobile fights can still take enemies out of it. You also won't know when/where the ball will proc exactly because of the random condition, so it's hard to place it just right.
Together, I don't think they have much synergy. Thematically similar yes, but not real functionality. You could get the benefits of both sets with similar uptime independently, no real need to pair them.