'Dormancy is more common in flowering plants than in gymnosperms, but some degree of quiescence characterizes living seed plants in virtually all the main climatic zones.'
Mapes et al. Nature, 337: 645-646 Evolution of Seed Dormancy
That's the simple answer. For the more complete picture there's a great review of seed dormancy in the New Phytologist journal: Seed Dormancy and the Control of Germination. There are different classes of dormancy (eg physiological dormancy, morphological dormancy etc) some of which evolved in the gymnosperms and some of which only evolved in the angiosperms. An interesting paper to browse if you want to see an example of how modern biology needs to combine information from many different areas, here information is combined from evolution, molecular genetics, physiology, biochemistry and ecology to understand seed dormancy and the control of germination.