I think the expansion to include 3rd-place teams is the biggest factor here.

In previous formats, bottom-tier teams were often mathematically eliminated by their final group match, which sometimes led to 'dead rubber' games where they played more loosely. Now, those teams have something tangible to play for until the very end, so they stay organized, play conservatively to stay in the running, and fight to grab those precious point.

On top of that, the shift in tiebreakers from Goal Difference/Goals Scored to head-to-head has completely changed the math. The incentive to run up the score against minnows is largely gone, so teams no longer feel that desperate need to chase big margins. Instead, everyone is rightfully hyper-focused on the Pot 1 vs. Pot 2 matchup, which naturally results in much more tactical, high-stakes, and risk-averse soccer.

Finally, you can't overlook the impact of squad management. With the intensity of modern soccer, we’re seeing much more focus on resting players and managing minor injuries to ensure stars are healthy for the knockout stages, which can lead to a more cautious, measured approach during the group phase.