I think because less info is required for them than a playable faction requires, the "reward" of having a banner is not as worthwhile. Besides, if everyone that made an NPC faction or took one up as manager of them had a banner, it would be a little disrespectful of them towards the holders of playable faction banner(s). I can bust out an NPC enemy in under 48 hours if I want (might time myself for one of these new ones) but Gunsight1 took a few years to make a playable faction. If I made a playable faction it would take a lot more effort on my part and the banner would be necessary as a "reward" for the time and energy I put into it, the time and energy it takes supporting new players, as well as the time and energy it takes to GM/ oversee plots.
Banners aren't there for GMs to know who to go to for what, they're there for players, especially new ones, to know who to go to for what. GMs should be aware enough of their surroundings to know what belongs where and to whom.