We Shia are required to pick a Marja (scholar) to follow as our religious guide for our life affairs based on who you think is "the most knowledgeable". Sayed Sistani is the world's most followed Marja in the Shia community. He's known for being politically neutral in basically every affair taking a stance against the melding of religion in politics, an alternative interpretation to Khomeini's position which creates the Islamic Iran in the wake of the Iranian Revolution.

However Sistani has broken this streak during the rise of ISIS in Iraq, declaring a Fatwa that mobilized millions of Iraqis to fight against the Daesh threat. People are essentially demanding him to declare another Fatwa for the current crisis as a way to mobilize average Iraqis into joining the war effort against the imperialists.

He's already provided a soft statement declaring that their is a social obligation to defend Iran a few weeks ago. However that is not a Fatwa to mobilize the masses, more so being a religious justification for aiding the anti-imperialist effort without explicitly calling to join the militant struggle.

Sistani was the student of the former largest Marja, Ayatollah Khoe, who was persecuted by Sadaam and placed under house arrest and surveillance like other Shia Ulema of the time. His hesitation is likely based on this formative experience of having to constantly keep a low profile rather then make political charged statements. It's careful, moderate, and very averse to actions that could recieve harsh retaliation or put people implicated by potential statements in danger.

Essentially, he is likely weighing out whether a Fatwa would lead to mass unnecessary casualties of Iraqis and Shi'ites globally, or if the existential nature of the fight against Iran requires such a sacrifice.

The theological implications of the decision are that he would also be held responsible for any actions done by his followers in the sincere name of his Fatwa.

To put it in less metaphysical terms, he's a 90 year old man who is fully aware that he will not be fighting in this war, so he's likely calculating whether or not he has a reasonable and moral justification to say that able bodied people should put their lives on the line.