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IMO this came down to Brand Gating. If a brand agrees to sell direct to Jet.com, Jet will not buy that brands products from other third party sources.
With the number of counterfeits which Apple faces, this is a HUGE selling point.
Seeing that Amazon is an open marketplace, and that creates price competition, they rarely lock down the ability for 3p seller to sell certain brands. This is changing a bit as of late, but not to the level which I've heard Jet will go.
Amazon wants prices low b/c it helps their customer base out. However, when those prices are low due to authenticity concerns, it can reach a breaking point.
Thus enter Jet. Beyond that - I believe you will see more brands that compete with Amazon joining efforts to fight back the dominance.