feline health female - why did they get so skinny? What did I do wrong?

I had a kitten/cat for 16 years. I also rescued an older cat who stayed with me for 5 years.  

The kitten had a normal weight. When I moved out of town, she got really active and very skinny. I was told she had hyperthyroidism. She got the usual iodine treatment.  

She never gained back the weight. I tried tempting her with tuna and yummy stuff to get her to eat. I tried different flavors, different brands, even senior cat food, to no avail. After a few years I could feel the individual bones in her spine. The vets did ultrasound tests, blood tests, and other tests. I was told she had high liver counts, whatever that is. One day she started having seizures, which progressed rapidly into grand mal seizures within an hour. The next morning, she cried out, so I checked on her. She had fallen off the bed. I picked her up and put her back on the bed. Then I could tell she could not see or hear me. She continued having seizures that day. I took her back to the vet the next morning, who advised me to euthanize her. She was 16 when she died and 9.6 pounds.

Next, I rescued an older cat about 8 years old.  Her records state 10 pounds.  

She too lost weight. She had good, clean teeth. She was a picky cat, only eating the gravy. 5 years later, the doctor said she was diabetic. Muffin started eating a veterinarian diet for diabetes. She died from complications of the diabetes.

Why were they so skinny and sick? Were they so skinny from being old? From being kept in a studio apartment?