Yeah if someone played the first day, loved it, and decided they're getting it, there's not THAT much reason to keep playing during the rest of the slam. There's a cosmetic reward for the intro mission but after that it's only some basic gear and weapons for reaching level 10/30. The kind of stuff I'll probably lose within a few hours of playing at launch. With launch being less than a week away, why grind levels in the slam and end up having to play all the early game content a second time?
With BF6 open test I played a lot across both weekends because I'm pathetic and I wanted the cosmetic rewards. But for Marathon I'm happy to wait one more week now that I've gotten a taste and got my settings etc. locked in for the 5th.