What is the appeal of playing Genestealer Cults?
A couple of things really.
One is that they're a legacy army, they were part of the original 40k and were one of the popular enough bits that they stuck around while things like the sharkmen got dumped even though they had no rules. The model range is even based off their original aesthetic. They've kept bits of the Shadow over Innsmouth fluff that they were partly inspired by
They're a major threat to Imperial planets and common antagonists in novels and RPGs. They appeal to people who've played Dark Heresy or Deathwatch or read Ciaphas Cain. Few things are as scary to a DH player as genestealers and their cults to the point that it's memetic.
Unique playstyle compared to most armies, at least in 7th, not so sure about 8th. They're the only army that had rules for being a guerilla force that didn't suck. They focused on short range firefights and close quarters assault with light infantry. Out of all the full armies they're the only one that lacks a tanky heavy hitter option. There's no terminators, no nobz, no land raiders or carnifexes, no banshees or incubi. The tankiest units they get are basically a weaker predator tank and a less powerful leman russ.
They were a good counter to Eldar bike lists in 7th because they could basically teleport onto the battlefield and stop the jetbikes getting a chance to run away like douchebags. With the tournament meta having become quite stagnant at the time that made them reasonably popular among tourny players looking for a meta buster.
They're quite the underdog army. Ragtag bunch of nobodies with shotguns facing off against organised armies with nothing but repurposed civilian gear and their genetic abomination family members. It's the closest you can get to playing an Underhive gang.