FISA was used because Stzrok had that in his tool box. McCabe and Ohr supported it as the easiest way monitor the Trump Campaign. Those three are the known Trump-haters and all worked together on this.
We will never prove it, but I am sure they also provided the information gathered via surveillance to the Clinton Campaign.
Whatever happened to all those “straight arrow” FBI agents that were eagerly waiting to come forward to spill the beans?