Well that's because many working class conservatives benefit from social programs. And being fiscally conservative means you support less of these social programs they'd benefit from.
Sanders economic policies that appealed to the MW and RB really is a response to many of them have worked labor jobs for large corporations and they feel tired of being abused by them.
But that's the same rhetoric Trump tapped into and they ultimately believedore because he was a tough talking, never back down, chest thumping, police praising outsider who was also a conservative by party.
There is something there for economic populism, but you're going to have to figure out a way to distance yourselves to offset the racial and religious aspects that conservatives offer.