In criminal law, there is an expectation of evidence/proof to establish probable cause. The reason that a hunch isn’t considered enough to determine probable cause is that a hunch can be tainted by bias and lead to violations of civil rights. The principal law that establishes the right of citizens against searches based on an officer's hunch is the 4th amendment to the US Constitution, in particular, the section that reads,

The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.

The main factor in determining if a search can occur, especially when a warrant is necessary, is if there is enough evidence to produce probable cause—which is determined by a judge. A hunch is not going to constitute the required proof to meet the burden of probable cause, so law enforcement officials would have the saying “hunches are never enough” to remind officers that they need to find evidence to pursue searches and seizures legally.

While it is true that hunches are never enough in terms of warrants, there is some nuisance and legal gray area when it comes to police stops. Courts have ruled through different cases (Illinois v. Gates, Terry v. Ohio, and New Jersey v. T.L.O) that the standard of reasonable suspicion can take precedence over probable cause in searches conducted in police stops or at public schools.

Reasonable suspicion is not necessarily a hunch, but it requires a much lower threshold of evidence than probable cause, and it does not require a warrant to conduct a search, which is an easier bar to reach. Some argue that the lowered standard of reasonable suspicion has led to police overreach, particularly in cases of racial profiling, but it is the standard used by the courts to this day.

In the context of criminal law, hunches are not enough because more concrete proof is needed in order to do things like securing a warrant or gaining a conviction.

When police are investigating a crime, it is not sufficient for them to have hunches.  They cannot get a warrant because they have a hunch they would find evidence of illegal activity in a given house.  Instead, they need to have probable cause that can be examined objectively by a judge.

When a case is brought to trial, it is not enough to have a hunch about the defendant's guilt.  The prosecution must have enough evidence to prove the case beyond a reasonable doubt.

Thus, in the context of criminal law, this phrase means that objective evidence is needed and that hunches are not sufficient.