Because the dataset is incomplete if even a single city doesn't opt into sharing data, or only provides a portion of the real data.
From the FBI itself:
> Pitfalls of Ranking
> UCR data are sometimes used to compile rankings of individual jurisdictions and institutions of higher learning. These incomplete analyses have often created misleading perceptions which adversely affect geographic entities and their residents. For this reason, the FBI has a long-‐ standing policy against ranking participating law enforcement agencies on the basis of crime data alone. Despite repeated warnings against these practices, some data users continue to challenge and misunderstand this position.