Probably because the bestem backrest mount covers the rear fender rail hole (at least mine does) and the bags require that hole for mounting. I'm going to have to drill and open that area up so I can use my backrest with my bags. I only use the backrest when I travel, so no use to purchase a new one. Shouldn't be a big deal.I have the Bestem Backrest (which similiar to the Honda Deluxe?) and it says on the Viking website these bags wont work with the Honda Deluxe. I was just wanting to check on here to see if anyone had a similiar setup before I bought them and they didnt work. Thanks guys.