A fruit or flower whose name you do not know is refused because it cannot be judged: if it is on the list it is wrong to eat, and if it is poisonous it costs a life.
Amuṇiya-nāmāṇi phulla-phalayāṇi, fruits and flowers of unknown name. The reason is procedural rather than doctrinal: the other twenty-one items can be recognised and this one by definition cannot, so the rule is not to guess. It is the item on the plate that stands for the whole method.
The twenty-two, laid by their reasons - Unknown fruit and flowers highlighted