Back before Sam Raimi walked away from making Spidey movies and the studio needlessly rebooted, Sony famously insisted that the director put Venom into Spider-Man 3 to bait the fans. They rightly knew that the character was a big enough draw to inflate the box office, though the execution left a lot to be desired.
Alongside the Joker, Galactus and probably now Harley Quinn, Venom has a special level of appreciation among fans that seems to transcend the comics he appears in. Just as the mythology of Punisher convinced studios to try THREE times to make a movie for him before Netflix managed it, there's some dark magic in Venom's brand that Sony KNOW sells. That's why he was the character chosen to launch this weird non-Spider-Man Spider-Verse they're building.
There's also a reason why Carnage was mentioned so heavily in rumours and why Sony consciously allowed the marketing to spoil the fact that there would be multiple symbiotes involved (thereby implying Carnage might be one of them) - he too is an established brand they could hitch their wagons to.
Sure, that was misdirection, but whatever works, right?