Because it’s cleaning up a symptom of Alzheimer’s (and other conditions where infections cause poor waste clearing/excess waste creation),
not reversing some known mechanism of Alzheimer’s.
If it’s waste to be cleared then a plateau would be the default assumption. Degradation past a point suggests something else.
That the medication is additional burden on already overwhelmed detoxification processes?
My best guess.
It could be a burden. Another intriguing possibility is that the waste isn't waste, instead maybe something necessary to trigger or run some other form of protective process.