Isn't the answer simple? Most cold cases have a very low probability of ever being solved. The rare exceptions make headlines, but I'd rather police dedicate finite resources to the hot cases with a better chance of getting solved. Those are also the ones where solving them actually helps protect public safety. Solving cold cases appeals to a more vague notion of justice, which is fine, but you're not likely to get robbed by the killer from a cold case in 1987.