Sometimes you don't want to keep getting funding from the same VC so that they don't acquire too much ownership and power in your company. To diversify its investor base, Snap would have to convince VC2 to pay a price that is higher than anything VC1 is willing to pay. Maybe it would work out and VC2 would be happy to pay a premium to get in the deal. But it's also possible that VC2, VC3, and VC4 all walk away from the deal because they don't want to deal with the defacto bidding war with VC1, who has more information and knowledge from being involved with the company from early on.
