Pinnacle doesn't have event related promotions either (unless you mean partnerships like with flashpoint, but that doesn't help players). Their model is have low margins and to attract as much volume as possible. Most sportsbooks cater more towards the casual players so they might have higher margins, but throw in promotions to help. Players considering Stake most likely don't have Pinnacle as an option, or aren't considering them anyways.

I don't know why you've said their bonuses are worthless; sportsbet's 7% profit boost is nice, but Stake has bonuses in other forms too, like the VAR promo (for example, in the Sheffield United vs Aston Villa match, players betting on Sheffield ML were refunded because of the goal not given) or the Guaranteed Payout one (players who bet on Man City ML against Chelsea were paid out as a win because Kevin De Bruyne scored).