1. There is a great liklihood there will be much blame to share, and they want the Hareidim to share the blame. Netanyau knows he’s in over his head, and wants to cover his you know what.
2. Heavy casulties will probably necessitate massive reserve callups and conscription of many people now exempt. If the rabbanim are pro-war, it will be easier to draft yeshiva students and frum women. It’s one thing to say “why should we die for your war, which we advised you against” and another thing to refuse to fight in a war you had endorsed (similar to the “chicken hawks” in the USA).