The short answer? Because all of her skills are designed to be constantly used in conjunction with each other to get any benefit out of them.
The long answer?
The fighting game archetype involves constant motion and actions dictated only by player skill and ability. A lot of her skills fit into this design - but, they must be used in conjunction with each other in order to get benefit out of them.
For example, one of the complaints is that Riven has two hard CCs on her 3rd Q and her W. The W has almost no range, and she has to shield or Q into range to use it - and then she has to maintain that to get the CC out of her 3rd Q and use the shield benefit. See, now that she's used the 0.75 second stun on her W, she's burned either her Q or E. This cycle repeats - she would only get decent benefit from her shield if she could engage with her Q or stun with W (or both, in the case of doublecasts) and her Q would only be useful if she could constantly weave it with her passive and stun both.
The end result? Her kit doesn't function without other components of itself, there's almost nothing that works well alone. Even her ult requires her to chain it with all of her abilities to get any function out of it...
Now imagine this with Mana or Energy. She'd get almost nothing out of it, and would either need to have numbers like Zed (which always tends to become a problem very quickly) or low enough costs that she'd end up like E-Max Bruiser Akali was (also known as ridiculously unhealthy).