Mobility and team organization.
In 6v6 you have far fewer heroes who are decent flankers… and so does the enemy. Which means that even if you do go Genji and walk around the moon to get to the enemy’s backline, you’re not going to find the enemy supports left basically unattended the way they are in 5v5. You’re gonna run up against a Mercy… and a Mei, a McCree and a Roadhog all peeling with the hardest CC you’ve ever seen.
That’s not to say that flanking was weak in 6v6. It just wasn’t the uncontested meta like it is in 5v5.
And if, in 5v5, you’re getting dove by the only tank while you’re flanking… that sounds like you’re probably not timing your engagements right. The tank should have far better things to do than chase a Venture around the map.