You want a legitimate source of income. If the bitcoins were obtained illegally or through illegal activities, you are fucked as soon as you try to cash out.

Even if you somehow manage to cash out and not get caught at that point, having large amounts of disposable cash with no legitimate source of income would also trigger some alarm bells.

Never forget that Al Capone went to jail for tax evasion, not all the other shit he did.

I think it’s more just because we’re early adopters and the first wave of refugees.

Yes, and because there are some little hurdles in the signup process. Having to select an instance isn’t really that big of a deal, but it will actually stop quite a few people.
The people who do make it through care or are invested enough to join and are less likely to shit the place up. It’s a self-selection process.