As a champion with a very assassin heavy play style, Diana loves the burst from Electrocute. She can proc the damage pre 6 with an [E]->[W]->auto thanks to the spells not having to do damage to count toward the proc.
Post 6 Sudden impact because a vital tool in blowing up enemy champions with all-ins.
Diana's assassin playstyle makes her heavily reliant on picking up quick kills and snowballing a lead and Eyeball Collection does just that.
Relentless Hunter is a bit of a stylistic choice. As an assassin, Diana usually wants to type to roam a bit and find a target to explode.
As as Assassin your damage pattern will typically consist of bursting someone 100 to 0 at the start of a fight, meaning you should remain above the threshold for Absolute Focus for the entirety of your rotation.
Scorch is a rune that heavily focuses early to mid game chip damage. If you're confident in your ability to land [Q] often is lane this can potentially put you in a great position to kill your lane opponent at 6.