Why is potassium more important than phosphorous for annual plants?

It's not. Grasses, both annual and perennial tend to require more phosphorous than potassium. Legumes normally require more potassium than phosphorous. Often times soils are short of phosphorous, not because there isn't plenty of it in the soil, but because most of it is locked tightly onto the clay particles, rendering it unavailable. In northern Missouri, an acre of land my contain 1,000+ pounds of phosphorous, but only 10-40 lbs of it is available to plants at any one time.