I feel like the public likes them BECAUSE they are so average.
Successful HF models usually have a unique weirdness to them, and are too odd looking for the public to admire. VS models are -in the public's eyes, again- of extraterrestrial beauty and perfection: unattainable.
Gig' and Bels are just pretty enough to be admirable but the public can't hate 'm too much: they're not insanely gorgeous and jealousy-inducing. Especially with their before & after pics slathered all over the web: chubby Gigi, pre-nose job Bella. Basically your everyday teenagers-turned-supermodels, a cinderella story many girls dream of.
not disagreeing that these girls are pretty average (their potentiality is debatable at this point considering they are so inconsistent with diet, etc.), but definitely feel as though their industry/ personal connections are really what have controlled and defined their level of fame/ "celebrity" not their level of mediocrity. Their social media presence is just enough to pique people's interest due to who they are dating or what Kardashian they are hanging out with this week. Companies pay big money for that sort of marketing and anticipate at this point them drawing in people more as social "influencers" disguised as "supermodels" ( a term that is ridiculously thrown around these days btw) because they are on the massive payroll of IMG. I'm sure there are people that disagree, but the Hadid's have an understanding of this and have worked PR to their ultimate benefit in order for people to think that they randomly achieved their level of popularity.