If the brocha for quinoa is an haadama because it’s not one of the five grains, then why is rice a mezonos?
Rice is in fact an exception to the rule. Chazal ruled that since it has similarity to the 5 grains, it’s bracha becomes mezonos. While other foods may also share some similarity, only rice was given this classification.
See Mishna Brura 208:25, Shaar Hatzion 31.