Because of aging
Part of normal aging is a decrease in renal function, the organs that filter out and excrete the harmful products in the body.
There is also a higher fat/muscle ratio to the muscles. This and a number of other changes requires a lower dose of any drug, especially digitalis, to be effective and non-toxic. In general, dose limit is 0.125 mg. Hypothyroid won't affect if it is corrected by thyroid med.