1. virtues motivate behavior
2. behavior is controlled by external factors (why? because social psychology)
3. virtues are internal
4. so, (by 2 & 3) virtues don't motivate behavior
Granted, a really rough characterization, but not too far off the mark. But just about any psych study will gesture in some way towards effect sizes. The environment is part of the cause for some behavior, but clearly not the whole story -- otherwise, the effect sizes for these studies would be ginormous. And they just aren't. But the studies are supposed to keep the environment the same, so variance in response is unlikely due to other environmental factors (or at least not solely to other environmental factors.) Is there some a priori argument why causes of variance can't be internal factors? It seems if the virtue theorist can get their foot in the door this way, it's a problem for most strains of situationism.
Plus, you know, individual variability in responses is a thing in psych. I guess the point of situationism isn't to eliminate internal factors but rather to show that they can't do the work needed to count as a character trait that might be developed into a virtue?