I changed 'sardines' to 'mezashi' and added a translation note. I couldn't find any info about them tasting any different, but maybe they're too dry for bread.
@hanih0 I can only assume some other component was part of the sandwich and didn't pair well with them? When I think "x sandwich" I don't just think of x + bread. There might be stuff like cheese, maybe tomatoes and stuff. But if it was just the fish and bread, being too dry or too moist could certainly pose a problem.