In my personal opinion there are many factors who contributes to get better
ROI in Agile than Waterfall.
First at all, Agile drives on Value whereas Waterfall drives on Plan.
Secondly, the most critical point in software development how to treat with
changing requirements? In Agile the impact is less than in Waterfall
because of you can add more requirements and prioritize as well any time.
Thirdly, deliver working software frequently allows to the client (Product
Owner) see what is going on and no to wait several months as in the classic cycle.
Last but not least, in Agile you work faster and more predictable way to the
clients and more responsive to market.
In fact, if your client realized that the software need to adjust he can do it
through the project.