The reason for this is mainly that since the printing of Ragavan, Nimble Pilferer, Dragons Rage Channeler and Murktide Regent the tempo archetype reinvented itself in being primarily blue-red and making Deaths Shadow not a necessary creature for tempo shells anymore
"The Dynamics of a Turn"
With planning, a careful reading of the rules, and a focus on strong underlying games, these promotional rounds can add variety, structure, and value to your regular sessions
Having Inititaitve can lead to additional payoffs via support cards like Imoen, Mystic Trickster and Safana, Alimport Cutthroat
Generally, the way tribal decks work (with a few exceptions I'll mention later) is that they want to play a lot of creatures and then cards that make those creatures' types matter
In the case of Gambit versus Berserker, the deck generally has enough other things to do on turn two, and the Gambit is just generically more powerful after turn twoespecially in the late game