Probably the usual reasons. Homogeneity, social cohesion... These are probably not universally correlated with low crime but they certainly tend to be.
EDIT: Downvotes? For this?
I'm not claiming that lacking the diversity is a good thing, merely that having extra options decreases cohesion. The more diverse the economic situation is (say, due to vast wealth inequality) the less cohesion there is. The more diverse the religions are, the less common-ground people have based on religion--as in most of European history. The more racially diverse a place is, the more tensions due to race (LA versus Helsinki).
Is it worth the hit to cohesion to have better diversity? I'd argue that, in the absence of external threats, it absolutely is. But living in a fantasyland where diversity doesn't come at the expense of cohesion doesn't help.
If you sort https://en.wikipedia.org/wiki/List_of_countries_by_income_eq... by "CIA Gini %" and skip the countries where there is no data in that column, then sort https://en.wikipedia.org/wiki/List_of_countries_by_intention... by the "2010" column, then squint a bit at the top ten or twenty countries in each list, they look kinda correlated. (Or co-correlated with a whole bunch of other factors.)
https://www.mitpressjournals.org/doi/abs/10.1162/00346530055...
”Among economic measures, inequality is generally a better predictor of violence than unemployment. But The Gini coefficient, the standard index of inequality, actually rose in the United States from 1990 to 2000, while crime was fallinh, and it had hit its low point in 1968, when crime was soaring. The problem with invoking inequality to explain changes in violence is that while it correlates with violence across states and countries, it does not correlate with violence over time within a state or country, possibly because the real cause of the differences is not inequality per se but stable features of a state’s governance or culture that affect both inequality and violence. (For example, in unequal societies, poor neighbourhoods are left without police protection and can become zones of violent anarchy.)
It seems to be somewhat inversely related to diversity but not entirely. As Megan has also written, Greece and Italy are also fairly homogeneous but don't have most of the social characteristics of the Nordics.
It's the inverse of diversity. Economic inequality, religious, philosophical & cultural diversity — make a measure which combines all of those things, then invert it. It'd at least make a start.