Probably the fact that they were. The 2.2 engines they were tossing into everything with four wheels really werent bad, it did get up and go pretty well by the standards of the day and got pretty decent mileage, but the rest of the car absolute pure junk. Knew enough people who had them when we were teens just starting to wrench on cars, and we were constantly having to fix... something. Anything. Freaking everything. My poor grandfather gave me his dead nuts reliable late 60's Dart with the beloved slant six, and bought home a brand new Horizon, and that rolling junkyard went back and forth to the dealer so many times he should have plopped his bed and TV into the service department waiting area and just moved in permanently. When he died my dad brought the car home to us and by the time that wretched beast was five years old and had 50,000 or so miles on it, it looked like it was 14+ years old and had 190,000. Of all the cars we ever had, that was truly the only one where I was seriously, honestly embarrassed to be seen in.
I know they sold a ton of them and I'm glad it helped their bottom line but the build quality of those cars was amazingly subpar even for that era.
They weren't that horrible. Most years they were the lowest price new car out there, they rode well, had a ton of space inside and comfortable seats. The 2.2 had a ton of power for that little car. The Holley carb sucked and didn't get replaced with TBI until 1988 and the door hardware felt super cheezy (the mechanism that operated the door latch and lock plunger) otherwise they weren't bad cars at all. Cheap, basic transportation.