Because that dam is primed to blow on its own:
Despite intense U.S. pressure to act to keep Iraq's largest dam from collapsing, Baghdad has done little to prepare Iraqis for the possibility of a burst that could unleash a flood reaching the capital and killing hundreds of thousands of people.
The government signed a $296-million contract with Italy's Trevi Group last month to reinforce northern Iraq's fragile Mosul Dam, but it has not announced any specific plans to try to rescue people in the event of a breach or instructed them in detail how to react safely.
A barrage of truck bombs and rockets could well collapse the dam and create a humanitarian disaster of biblical proportions that will require an all-hands effort by Iraqi military personnel to cope with, ending any thoughts of a Mosul offensive for years to come.
For that matter, this kind of unconventional attack on Iraq would halt the advance in Anbar:
Iraq's counter-terrorism forces backed by army troops and U.S.-led coalition air strikes advanced towards the western town of Hit on Thursday in an attempt to dislodge Islamic State militants, the military said.
A senior officer from the counter-terrorism forces, the elite U.S.-trained units which led the recapture of nearby Ramadi three months ago, said his troops were one kilometer from the town center, 130 km (80 miles) west of the capital Baghdad.
If the Iraqis keep this up, they won't need a Jordanian advance from the west to clear Anbar province (although to kill and capture ISIL forces rather than drive them back, a Jordanian attack would sure help).
Anyway, attacking the Mosul Dam is what I'd do if I was a nutball ISIL jihadi military planner in Iraq, anyway.