I agree... there is very little incentive to running these considering the difficulty, which is why I typically avoid them.

Dragons still get farmed because of the ingredient drops, as a result they're relatively populated. It would be great if harrowstorms could get something similar to draw the crowds. Maybe increase the chances slightly for the grey satchels? Give us a chance for a treasure map?

Like @BretonMage said, most of the problems ESO has with its content is a lack of evergreen rewards to keep people engaged indefinitely. Dragons always remain relevant because the alchemy reagents they drop are used for PvP; this is an evergreen reward. Motifs and overland gear you can get literally anywhere else have a short-lived usefulness in the broader gameplay loop. Once most of the player base completes a motif and the furnishings, like say during an event, there's not really any reason to go back. This is why Bastion Nymics are unpopular, because it was more effort than a Harrowstorm for pretty much the same trash you got from any other incursion. Ironically, Volcanic Vents seem to grant better gold via treasures for the time spent on them than other 'events'.

Personally I stopped pursuing Motifs and furnishings altogether because I have so many now that it's just pointless grind for something I'll never use.