Because evidence gets lost and degrades over time. Witnesses forget things and die. The longer ago a crime was, the harder it becomes to prove who did it (and to provide an alibi, which, while you are technically presumed innocent, is still important to keeping the wrong people out of jail)

Plus there is the more philosophical question of how long someone should be held responsible and if, at any point, it becomes less about correcting behaviour and more about vindictiveness. If someone committed a minor crime at age 17, and they are now 65 and have never once committed another crime, is society served by holding them responsible now? If prison time is supposed to reform someone (which is what many believe), does holding someone responsible for a minor crime they committed 40 years ago help anyone? Obviously there are crimes that are so immoral we should always keep people responsible (like murder) but for a lot of minor things, there is many people who feel that at some point it becomes irrelevant to prosecute.