I haven't seen a Church's in ages, I always liked it better than KFC.
As for In-n-Out, its popular for a few reasons (even though most folk that try it now are not super impressed):
- Its cheap, but still pretty good
- Its got a "secret menu"
- It's deeply connected to car culture in California (many locations host car shows every weekend)
- People that grew up in California connect it very strongly to their childhood
- The fries!
So I see why people from California love it, and my brother who lives in Vegas really likes it (he's pulled in by the "secret menu") but I also can see why most everyone else is underwhelmed by it (myself included).
If I had one close by I'd eat there occasionally, but there's just too many other places that are just as good.