Because

   Why is my sweet mother, who earned a doctorate in education by going to night school while bringing up six children and working full time, a Scottish/country/clog/square and ballroom dancer, a Master Gardener, who drank tea every day and loved puzzles and ate her weight in organic fruit and leafy green vegetables, went for daily walks, attended a weight lifting class, took classes to learn new skills, volunteered right and left, supported her church, had excellent stress-coping skills, watched her salt intake, subscribed to various hospital health letters, prayed every day, drank in moderation, helped out with civic clubs, had family living with her, visited the elderly in nursing homes, read fiction and nonfiction, stayed in touch with old friends and with current events, was curious about everyone she met ... why is this 91-year-old woman ending her days immobilized by dementia?

   She smoked for maybe 14 years as a young adult, is that why?

   Her sister had Alzheimer's and her family has a few reports about old ladies in Ireland wandering in their nighties ...

   She was subject to debilitating migraines before menopause ...

   She surrounded her head with a cloud of hairspray almost every day for four decades ...

   She stayed awake half the night working on lesson plans or IEPs and once became so chronically sleep deprived that she feel asleep into a bowl of soup. Is sleep deprivation why?

   She hated getting little basal cell skin cancers and so she hid from the sun for about a decade until she had osteoporosis and then took a course of Fosamax to fix her bones ...

   She spent a decade in agricultural areas of southern Texas with their water systems ...

   Her whole household didn't trust Sherwood water and so only drank home-distilled water and when that machine broke, they only drank juice, milk or soda pop. Sometimes they dehydrated themselves into odd health conditions ...

   She loved ice cream, is that ... ?

   My current theory is that she dehydrated in 2010 (possibly earlier) and had some small strokes that killed parts of her brain. For certain in the summer and fall of 2014, when all of the sudden she had atrial fibrillation, she was chronically dehydrated and prone to UTIs, which only deepened her confusion, making her even less inclined to drink water. But she was already demented then.
 
She seems to be about as demented as she has been for the past four years — it hasn't worsened. Only her body is radically weaker now.

   At Thanksgiving 2013, a friend who was sharing dinner with us was horrified to see Mom poking her vitamin D and calcium pills into the serving bowl closest to her plate. She always poked her pills into the food on her own plate so she could hoover them in without thinking about them — but this time she was confused by the extra dishes around her.
   Before she became that bent, silent biddy, Mom might have poked pills into a serving platter absentmindedly, while distracted.
   One time when she was about 70 and still vital and dancey, I watched her cross the kitchen to get milk for her tea, open the fridge, take out the milk, head for her tea cup but stop at the stove and pour the milk into a saucepan of boiling spaghetti. She had a lot on her mind that day, a lot; still, she would make little processing errors all the time ... use the wrong word, call her children by one another's names, forget where she put her hat, hit the accelerator instead of the brakes ... she'd joke that she needed to eat more kale — and then she would eat kale.
   Did she have a defective brain, is that why she has dementia now?
   Did I mention that she earned her doctorate by going to night school while working full time and raising six kids?

  I have no proof that dehydration caused her dementia, and I have no medical education to help me assess the quality of such proof if I did have it; but still, dehydration-caused stroke: That makes sense to me.