Our intelligence fails in Iran for the same reason Indian intelligence fails with us. They have far deeper cultural and religious ties to exploit than we do.And so what's stoping Pakistan from using the same tactics in Iran?
Brotherly love and affection? That's been propagated by islamabad for all countries that are Muslim. Because they can never be think of harming Pakistan.
This wannabe Persian empire needs to be put in its place or else they might start their activities terrorism in Pakistan (increase it 10 fold from current mode)
It doesn't help that lots of Pakistanis all the way to the top echelons of our society fall over themselves to betray Pakistan to Iran. Then there's fear by top command of needing to beef up the western border at cost to the eastern one, if things start to get out of hand. We do however have the Jundallah as counter to their BLA assets.