I'd say they cant produce a low enough signature with the nacelles on the bottom...

benmowa on Jan 4, 2019 | parent | context | favorite | on: Boom Supersonic raises $100M, aims for 2019 test f...

I'd say they cant produce a low enough signature with the nacelles on the bottom as currently displayed. see all aircraft in this nasa clip from April 2018.

https://www.youtube.com/watch?v=DPd0OHrmCBc