Most of us want to kill weeds like dandelions when they are most visible: in the spring. Spring flowering weeds are visible because ... they are flowering. That tells us that the plant is in a reproductive cycle, using available energy to produce flowers and seeds for new plants.
Most of the flow of water and food within the plant goes upward to sustain the production of these flowers. So it follows then, that a larger percentage of the herbicide we apply in the spring also goes upward and not down to the root. That's why the flower head wilts long before any leaves show signs of damage.
It is much more efficient to apply broadleaf herbicides in the fall, when the plant is translocating water and nutrients (along with the herbicide) down to the root system in order to get ready for a long dormant season. This is particularly relevant for perennial weeds like dandelions.
While we do make some herbicide applications in the spring, it is mostly cosmetic. Our most efficient use of resources is to make these applications in October, when we get a much better "kill ratio" on perennial weeds by attacking the root systems.