There's all kinds of shows.
Breaking Bad is in a real class of it's own, Better Call Saul has a long way to go but it's up there but those are really shows that actually do awesome jobs of mixing humor and feelings together to involve you in a story.
The Walking Dead is great too but in a different way.
The Walking Dead IMO is just one of those shows that ties into a meme that fits for it's time.
It doesn't have to be as good to be even more entertaining because it's plugging into something rather primal that also coincidentally happens to be in the smack dab middle of peoples consciousness.
The Walking Dead is great because it is what it is. It's a show that let's people feel things they already feel and escape into a reality that does something to play to their more primal urges it is amazing at recapturing basic humanity that I think many people feel modern society has too long suppressed.
We weren't meant to live in the modern world, this show simply is one massive mental exercise in allowing people to fantasize that they don't and it's really that simple.
For that reason the writing does not need to be on par with the best shows and it's certainly not but it wins a lot of points by default for tapping into something on a grander scale.
It's popularity has nothing to do with writing, although at times it can be very good and at times a big fat meh to bleck. That doesn't matter because they are riding a wave at the right time.
It's a rare thing to have something like it. It's very basic human nature on display without being submerged in modern social morass. It's got Hero's and Villains and it strips away the modern veneer that often obscures them.
The Dead are a metaphor IMO for most people on earth, consuming mindlessly without thought. Everyone is infected but a few people are still vibrantly alive presenting a stark contrast.
The modern world at times can be suffocating and can strip away raw humanity. In the Walking Dead world while it might suck, all that vibrancy is the only thing they have left so it shines all the brighter. You literally can only screw that up so it's a much lower writing bar that they sometimes do a great job with and typically sometimes they toss in duds.
The worst thing that can happen to you isn't death, it's becoming a walker which is a metaphor for literally walking around dead. In the show it's better to be dead, really dead, anything but that.
That's why people tune in.
JMO.