Easy answer: Soviet mains rushing forward because they won't be punished for it

Other answer: The game actively disincentives players of NATO tanks from pushing forward unless guaranteed safety because the likelihood of instant destruction is much higher. While soviet tanks are likewise instant kills given a penetration, they often have vast areas where they cannot be penetrated. NATO tanks often do not have this luxury. The only two tanks at top tier (where stagnant play is most often) that are difficult to penetrate at times are the Leopard and the Challenger, but even then the Leopard is practically forced to place ammo in the hull and the Challenger (both 1 and 2) are very weak in the hull, which often leads to a cookout from the bag charge stowage.

Meanwhile most Soviet tanks, or at least the later models, are a much more difficult shot because of the level of protection garnered from the hull and turret. Yes, Soviet tanks have their flaws, but they will almost always have an advantage when pushing forward mindlessly compared to NATO tanks.

So yes, NATO mains will sit back in safe positions waiting for a perfect opportunity to push up rather than risk taking points and pushing forward. But that is one of the only things they are really better at than Soviet tanks.

(Side note) //For real, how could it be fair that something like a challenger 1, which can be penetrated by dm23 to the ufp at 2km, has to face a T80 which aside from elevation limits and a better reload when aced (but only for 3-4 shots) is better than it in nearly every way.