Why isn't manage section for data modeling and relationships not as good as Power Pivot? Power BI for Office 365 (Excel 2013) has a very good data modeling feature using a seperate Power Pivot window and easy GUI and access for data modeling / relationships. Creating measures and calculated columns in report section makes it quite uncomfortable as output cannot be viewed unless dragged to report.
My guess it's because the focus was on developing NEW features the rather than replicating old ones. I think it's a safe bet that we will see many usability improvements as the product matures.