I think because they where "jealous" of what FB is! I don't think they can bring revenue or at least what they expect. At the end they would transform their product to be more Facebook-ish, like others already do. For me (mid 30s) snap is for teens and I don't think teens can spend a lot of $$ in order advertisers to continue spending money for the users.
Investors, founders and employees wanted to have liquidity on their stock. Secondaries market is very limited.