My theory is that big private investors cannot buy enough of this stock to warrant attempting a position.

A bigger investor, like Stifel, would want to have many millions of shares to make the investment worthwhile. If anyone attempted to buy that volume of shares the price would rise very fast probably to a point beyond what they estimate it is worth at this juncture, before they were able to buy more than a million shares. They can't participate in a PP because those are now all closed to outsiders.

So, even if someone were interested in this stock, and though it was worthwhile there doesn't appear to be an ability to buy in substantial volume.