It's open only to accredited investors, because it is legally risky to accept investments from non-accredited strangers. It's commonly accepted practice to raise friends-and-family money from your existing network, but if one of those investors can claim their contact with you came from a solicitation to invest and not from some preexisting relationship, they can present legal risks down the line.

It's my vague understanding that the real issue here is "weird" non-accredited investors make the legal part of raising a VC round much harder.