I always wondered why I had ZERO volunteers in a bed where many Stupice tomatoes fell and rotted.
I figure it was either that I did some raking and turning that fall or the next spring, had some bark mulch, and that bed is a little close to the water table in winter ... they may have rotted in the soil.
Or the slugs are consistent enough to mow down EVERY volunteer seedling that isn't a recognized weed.