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

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

More than 6 million Americans, and millions more worldwide, have Alzheimer’s disease, the most common cause of dementia. There’s no cure, but Leqembi clears a sticky brain protein called amyloid that’s a key indicator of Alzheimer’s.