Actually, there are a few possible reasons for this.
1. We are showing the world that we are the good guys, knowing Iran will show their true colors.
2. We could be stalling for time ourselves in restocking our weapons or even designing a newer campaign to get the bastards.
3. There may be moderates in Iran we are talking to who need the time to get their own Republican Guard, who are currently running the country.
I hate to see it but it may be that the only way we really get them is for both Israel and the US to send ground troops to certain areas. We need to get the Republican Guard.