Why do state laws put an expiration date on sex crimes?

On April 27, 2016, former U.S. house speaker Dennis Hastert was convicted of breaking banking law, but crimes to which he confessed in court — sexually abusing multiple high school boys in Illinois while he served as their wrestling coach nearly four decades ago — would never be prosecuted. Their statutes of limitations had expired.
A year later, Illinois Attorney General Lisa Madigan declared the state had removed the criminal statute of limitation for sexual assault, aggravated sexual assault and aggravated sexual abuse against children. She credited Illinois’ passage of that legislation with the “powerful and courageous testimony of survivors,” many of them speaking publicly for the first time after years of silence, anger and shame.