I would imagine they like the flexibility of having an on-use effect instead of a proc-chance. Different fights can have different results for a proc-based trinket vs one with an on-use effect. For example, if it's a fight with a lot of movement, something that's proc-based may end up being less helpful since it can proc at terrible times. Not to mention the Mirror's mastery proc requires you to pay close attention to its timing to make the best use out of it with shadow orbs/empowered shadows.
Of course, I am not either of them so I can't really say what their reasons are for certain, they will have to answer that for you! :)