It's the first drug shown to slow Alzheimer's. Why is is it off to a slow start?

To a certain degree this is something expected and likely to happen, the disease has been in study for many decades already, and many previous attempts to treat it have ended in failure (with at least some better understanding being produced) it is not rational to expect this to change completely and for the first drug to show benefits to be absolutely effective, instead patients got something that is better than not being treated but that does not even completely stops the disease and that comes with its own risks. A slow process of adoption is the responsible thing to do, with a little bit of luck better options will appear soon and their use will spread with higher speed.

The FDA is reviewing another potential treatment that targets amyloid protein, Eli Lilly and Co.’s donanemab. Eisai is planning to seek approval for an injectable version of Leqembi that would be easier to use. 

Credit to the FDA for backing this and other drugs like it in the first place.

Apparently one contributing cause of alzheimer's is aluminium. A UK scientist by the name of Chris Exley has found that regularly drinking silicon-rich mineral water over time helps to safely draw the aluminium out of the body. Whether true or not, I don't know but drinking mineral water never hurt anyone or depleted their bank account too much.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6159653/

Apparently one contributing cause of alzheimer's is aluminium

Exley is one of the worst references you can get for the claim, he has been found repeatedly spreading false or misleading information because he is fixed into making aluminium the cause of every health problem he focus on, his invalid methods and faulty data already have caused the retraction of reports he authored and it is considered pseudoscience. He makes absolutely no effort to refute the scientific evidence that contradicts his beliefs.

Credit to the FDA for backing this and other drugs like it in the first place.

That is still not correct, as the other comments helpfully clarify for you, the FDA role is not supporting the drugs development or testing in any way, it just approves them once all the hard work is done by the companies.

But researchers say there’s still a long road ahead. It’s not clear what causes Alzheimer’s disease, and many think combination treatments will be needed to stop it.

Researchers are getting there, along with the backing by the FDA.

Researchers are getting there, along with the backing by the FDA.

Since the FDA provides zero help with the development of new drugs the researchers are doing their job by themselves, if anything in spite of the FDA.