In other words they got tired of enforcing the law.
Or, state troopers decided it made more sense to spend their finite resources on nabbing those who are speeding excessively, thus collecting the higher fines.
The same thing will happen with this new limit but, not immediately. At first, there won't be many people who increase their actual speed by 5 MPH; so, troopers will not "allow" for the usual 9 MPH cushion. If they did, they wouldn't be writing enough tickets. As the driving public gets used to and adjusts to the new limit, nearly all will return to our previous patterns where enough of us are going 10 MPH over the limit to keep the state troopers busy writing tickets.
The Iowa State Patrol has already told the media the new speed limit will be more strictly enforced. This is why. It's either that, or sit on their thumbs until people get used to the new limit.