Many of these requests come from relatively new "geofence" warrants that ask Google to hand over location information for each device that passed through a given area over a certain period of time. Google will anonymize the data at first, but it will provide names and other sensitive information if the police believe it matches the behavior of a suspect or witness
Location History's existence is not a secret. It's been available since 2009, and you have to grant permission before Google starts collecting data. However, people do not necessarily realize that Google keeps the info for an indefinite period, or that history is sufficiently detailed to provide a picture of street-by-street movements to investigators.
More importantly, there are legal and technical concerns to law enforcement's increasing dependence on Location History. The Fourth Amendment requires limited searches and a probable cause, but there has not been a formal ruling on whether geophysical searches meet that standard even with the narrowing process Google uses. Not every police department will properly seal identifying data, either, potentially exposing innocent people.
And then there is the unreliability of the info ̵