Same with my sister-in-law and her husband. They weren’t about to watch some zombie show...they preferred good character-driven drama. We told them, yep, and that’s exactly what TWD is.
They got hooked and ordered every season on DVD. When one was a little late arriving, SIL called up, wanting frantically to borrow ours, lol.
IIRC, the first season was a ‘cable hit’ because it got around 3 million viewers. That is great for most cable shows.
Each season, the viewership grew. By about season 4, it was deemed a major TV hit, as it was drawing viewer numbers that made the 4 regular networks drool.
Now, it is about the 2nd or 3rd most watched program on night cable and network TV. NCIS maintains the highest total viewership numbers and TWD or Big Bang Theory come in a close 2nd.