Why can an AC circuit breaker not be used in DC circuits?
Direct current is more difficult to interrupt because it is continuous. Alternating current alternates polarity so it crosses zero on its own which helps to extinguish the arc produced across the breaker contacts when they open. The contacts doesn't get this help if it's DC so the contacts have to be hardier.

Interrupting DC with an AC relay/breaker could cause the contacts to weld shut or become degrade the conductivity.