The issue of elders who were once reasonably clean adults refusing to take showers and wear fresh clothes is one that is far more common than most people think.
Sometimes the issue is depression. If we have a parent who no longer takes an interest in staying clean or wearing clean clothes, it's wise to look at depression first. A checkup with a doctor is a good idea, especially if low energy is also part of it, or if they just don't care about anything at all. Depression isn't always obvious to an observer.
Another factor is control. As people age, they lose more and more control over their lives. But one thing they generally can control is dressing and showers. The more they are nagged, the more they resist. "This younger generation is trying to take over everything. Well, they aren't telling me when to shower, that's for sure. Besides, I'm just fine!"
A third issue is a decreased sense of sight and smell. What your nose picks up as old sweat, they don't even notice. Not on themselves. Not on their mate. Their senses are not as acute as yours, or as theirs once were.
A fourth cause is memory. The days go by. They aren't marked with tons of activities as they were when they were young. If there isn't something special about Wednesday, well – it could be Tuesday or Thursday. They simply lose track of time and don't realize how long it's been since they showered.
Also, working in with memory is the fact that many of our elders didn't bathe or shower every day when they grew up. We now take daily bathing for granted in this country, but when our parents were young, a weekly bath was likely more the norm. They may have gotten into a more frequent bathing habit in their last decades, but their brain is taking them into the past. Once a week, it's bath time. Then, they forget what day it is, or even forget when they last took a bath or changed clothes. Time just slides by.
Another big issue can be fear or discomfort. Fear of slipping in the tub. Discomfort trying to get in and out. More serious is when a person with Alzheimer's or dementia is in the bathroom and doesn't understand why there is water running on them, or believes the drain that may suck them down. They just don't understand what you are trying to "do to them."