There are a multitude of reasons for forgetfulness in seniors:
Co-morbidities i.e. other disorders may factor in, like Diabetes Mellitus, and Hypertension.
There may also be other medications like opioid pain killers (a substance akin to morphin), which can contribute to loss of memory.
Occasionally NPH (Normal pressure hydrocephalus) is diagnosed as the culprit for loss of memory.