Why do we need HANA for SAP Fiori Analytical Apps

Hi Experts,

There have been many documents and even SAP documents say For Fiori Fact Sheets and Analytical Apps there has to be HANA installed in customer landscape.

My question is why can't we use these Analytical and Fact sheet apps on top of SAP BI/BW?

Please give me the reasons why we have to go for HANA though the customer already has SAP BI/BW?

Best regards,

Jashua