Why doesn't Victor protect his wife, Elizabeth, from the monster's attack on the night of their wedding?

Because he misunderstands the monster's warning and he thinks that Elizabeth could protect herself.

Explanation:

During the course of the novel Victor and the monster have become irreconcilable enemies, leaving no truce in the intention of destroying each other.

The monster has committed several criminal acts in pursuit of his revenge on his creator but Victor has stopped acting rationally by letting himself be carried away by a strong mental instability, disregarding the monster's ability to find and punish him.

Elizabeth represents the peace of mind that Victor needs, the selfless altruism of unconditional love but also the rational strength that the creator does not have.

Actually, Victor is desperate in the village of Como where they had retired to celebrate their wedding night despite the gloomy atmosphere and the bad omens.

Victor Frankenstein becomes more and more anxious because he is certain that on that very night, he or his creature will die tonight.

Elizabeth, seeing him so agitated and indisposed, begs him to tell her what he is so afraid of. Victor tries to comfort her, but cannot answer her question, and only asks that, in order to prevent his beloved from seeing the monster, she should retire to her room while he watches the residence and the surroundings for any trace of the monster.

Finally, Victor hears a horrible scream coming from the bedroom where Elizabeth is resting, but it is already too late: Elizabeth is lying strangled and Victor is aware of the great mistake he has made.