Jet's last valuation was $1.6bn and it was, and continues to, burn cash like crazy. Investors aren't dumb, they see that.
So they add a terms to their investment whereby they guarantee a return on their investment. For a company that needs hundreds of millions of dollars and is trying to beat Amazon? Let's say 3x.
That means that if I put in $100m dollars at a $1.6b valuation, then you sell at a $4.8bn or above, then everything is fine because I got my 3x. If you sell for less, as they did, then I get my $300m first, before anyone down the line. That's called liquidation preference.
Jet was burning capital so they likely had to make a lot of concessions in fundraising. Given that their estimated total funding is around $800m, there isn't much else to go around.
You'll notice that some employees and the founders got around $300m worth of Walmart stock. Why would they need that if the company just sold for $3bn? Because the sale likely netted them next to nothing.