okay, so if it's accepted that chromatic orb and inflict wounds are balanced spells.
expected damage on-hit (edoh) is about 15, with the chance to crit for a first level spell.
Eldritch bolt thereafter gives edoh of 6 (or 6+cha)
Witch Bolt has an edoh of 7, with some number of persistent damage.
So, it's costing a spell slot for a higher risk against one target, with no real payout. Especially in a "system" where alpha damage is king. And as-is, puts us about 10 damage behind spells of it's class.