This is not about BO Webi reporting.It applies on all type of reporting.There are multiple factors why usage is very less but correct person to answer on this is end user.
Some are based on my experience.
Adoption of new tool
Users awareness about new tool and benefits
Technology sync:One example is you are on BO 3.1 but SAP released BI 4.2.
Frequent analysis not happened of reporting usage.
Users training
I would like to leave these points now on to you so that you can add more from your experience.