It's the item changes really. I hate Riven too, she is the definition of bullshit champion, and it seriously irks me when someone tries to ocnvince me she takes any skill at all. Yes, I've tried her, animation cancelling and just remembering to get off her empowered autos in between abilities isn't fucking difficult!
That however is just her kit being overloaded, so without the items to back that kit up she can't really do much. Brutalizer being removed and Ghostblade actually being expensive, on top of the change to Tiamat really hurts her early game, as well as cheap total-armor pen not being available anymore and the rest of her items also being more expensive.
Next to that is the meta shift to a more ADC focused one, so more peel and more backline. It's much harder for Riven, or any champion like her really, to get into a teams backline to kill the carries now, since there's typically more ways to peel her off and cc her on top of the ADCs being far more powerful in general.
So yeah, I loath Riven and would not be upset about her being nerfed to the point that I never have to see that skilless fuck in any game ever again, but I don't expect anything like that to ever happen so I think the buffs are actually needed for her to be viable outside of bronze. Honestly she'll either need more of a buff then what she's getting, or some item changes/additions will need to be made in order to give her and other champs like her the power to be viable again.