Walking Dead isn't that good IMO, the pilot was amazing, but after that...
It gets good ratings, but so did Smallville, which lasted 10 seasons.
You can't compare the two. At all.
Smallville, as a network show, should've been getting far more viewers than it did. The only thing that saves it is CW's lower ratings bar. But it lost viewers literally every year it was on. The final two seasons were under 3 million viewers per episode.
The Walking Dead so far has not only had to fight against being a cable show... on a network nobody watches, even. But it has increase ratings every season. The premiere this year had 10 million
This is on a network where it's other critically acclaimed shows are in the 2-4 million range.
It's crazy, and you are wrong here.