Diabetes is well known to be a massive risk factor for dementia - so no, people are not ignoring this!?

More broadly, diet is a frustrating modifiable risk factor - there are lots of studies suggesting the (somewhat nebulous) "Mediterranean" diet is protective, and beyond that, omega 3s.

But human diet is uncontrolled, highly varied, and reporting is difficult to obtain and unreliable (people don't want to admit what they ate), so a lot of these data are seen as interesting, promising but unconfirmed.