I've always suspected that was part of the reason- it's simply far easier to defraud small fry who don't have the experience to spot fraud or the resources to prosecute it.
Legal and diligence costs on most illiquid investments are tens of thousands of dollars. Anyone investing small amounts (a) can’t afford the diligence, (b) probably doesn’t know what they’re doing and (c) can’t afford to lose even that small amount. That attracts a specific variety of fraudster (see Bitconnect). We’ve seen this happen every time we break the wall between markets with high and low information assymetry.
Unscrupulous issuers have historically targeted less-sophisticated buyers who are less likely to be experienced investors - and also less likely to retain their own counsel to review an offering. Hence the accredited req.
In business investing its the opposite. They really do think they stand a chance at making millions even when the odds may actually be worse than gambling in some cases, especially if they fail to do their due diligence.
So maybe instead of regulation we simply need a legal, notarized document signed for every investment that states "I am aware that I stand a very high chance of losing all of my money and relinquish my rights to sue anyone involved unless outright fraud has been established." Probably still wouldn't work, but its worth consideration.