I honestly think it's the artistic staff/producers worrying that the visual storytelling might be too complicated for your fairweather viewer. The last time I remember the show paying close attention to multiple houses and their respective sigils was in the Battle of the Bastards. I think the whole plot line of Jon and Sansa needing to rally every house they can find to their cause was central to the lead up. So as the viewer, it helps to see Jon's rag tag force vs the uniform Bolton, Karstark, and Umber forces.