Why can't you use a 30 amp overcurrent device on a 12 awg thhn conductor?

12 ga. Wire should be protected by a 20 overcurrent device. First of all it's code (NEC). In addition to that, wire will overheat if exposed to heavy loads which can break down the insulation, melting it and exposing the wire in some situations. Obviously a bare wire is not safe in any respect when it is a current carrying conductor. It's simply not wise or allowed.

why can't you use a 30 amp overcurrent device on a 12 awg thhn conductor?
You can, in very specific and rare instances, but that is not a topic for a DIY forum.
NEC 240.4(G) is the code reference here.

Aljames, why do you ask?

Stan is very correct, only in very specific and rare circumstances is it allowed, and I want to be sure this is understood.

Generally speaking, and certainly in any DIY situation, the answer is no.