Why is the either-or fallacy an informal logical fallacy?

The either-or fallacy is an informal fallacy because it relies on presenting a limited number of options as though they are the only choices available, despite the fact that other alternatives exist.

It is a flaw in reasoning that affects the completeness of an argument, rather than a violation of formal logic rules. As such, it falls under the category of informal logical fallacies, like equivocation fallacy and slippery slope fallacy.