Aroldis throws a very straight fastball and hitters just aren't as blown away by 100+ MPH like they were even 2 or 3 years ago. Almost every team has several guys that throw 97+ and a few have guys that throw 100+.
This all adds up to hitters being better prepared for him and his very straight and very hittable fastball. He gave up more than his fair share of hard contact last season.
I feel like adding an effective changeup or curveball to his arsenal would work, but evidently his entire life his coaches have said "Nope, that fast ball is good enough."
And no, I don't think his slider counts as a second pitch. The only reason it was "effective" at any point in his career is because everyones waiting on a 100 mph heater