Stormcast are essentially models in 50 mm scale. Especiallythe more recent ones with human proportions. This makes them easy enough to start painting, while not as monumental a task as King Brodd's people. Their design is also quite easy to paint, and some of their models genuinely look good.
Dawnbringers have smaller details, (not to mention things like the Celestial Hurricanum, which is awesome) and the future of some bits of the COS range (basically everything that isn't Dawnbringers and adjecent heroes) is uncertain. You really don't want to have people that just started having their faction stripped or canned.